Freshwater pearls especially are often dyed. Any very bright colour or deep colour is certainly dyed – there are no natural green, blue, red, purple etc freshwater pearls, at least so far. There is a wide choice of organic and inorganic dyes available and they are all permanent. Gold south sea pearls can also be dyed to deepen their colour (and value)
This is the brand name given to a range of large bead nucleated freshwater pearls from one leading Chinese supplier. One strand of these pearls achieved £1/2m at auction. The pearls come from a Hyriopsis hybrid between Hyriopsis cumingi and Hyriopsis schlegeli.
Nearly every pearl available anywhere in the world is farmed – cultured. Pearl farms tend to be stunningly beautiful places.
Fake. A false pearl bead manufactured by coating the inside of a hollow glass sphere or the outside of a solid glass or plastic sphere with a pearlescent coating which is sometimes pearl powder. Faux is a fancy word for fake. Also called shell pearls. They are of course perfectly round in shape, with great lustre and even colour. White shell pearls are very white, which is a give-away. All fake pearls feel smooth when rubbed on the teeth and the drill holes tend to be larger.
There is a young but growing pearl industry in Fiji, and the pearls produced have a huge and stunning range of colours.
A pearl grown in a freshwater river, lake or pond margaritifera mollusc. Often more irregular in shape and more varied in colour than salt water pearls freshwater molluscs are nucleated by creating a small incision in the fleshy mantle tissue and inserting a piece of mantle tissue from another oyster. This process may be completed 25 times on either side of the mantle, producing up to 50 pearls at a time. The molluscs are then returned to their freshwater environment where they are tended for 2-6 years. The resulting pearls are of solid nacre, but without a bead nucleus to guide the growth process, the pearls are rarely round.
Gamma irradiation turns the nacre of freshwater pearls very dark, and often also imbues a metallic lustre with rainbow orient. Strangely, it has no effect on salt water nacre but will turn the nucleus dark which shows through the layer of nacre, making the pearl look grey or blueish There is no danger of radiation contamination from irradiated pearls.
or popcorn pearls have a knobbly surface which resembles..popcorn. This granulation is often mixed with patches of high lustre surface. When the Chinese freshwater pearl business was starting up most of the pearls were, at least to some degree, popcornish and oval in shape. They were called Rice Krispie pearls.
Gold Leaf Pearls
This is the name we give to the extremely beautiful lustrous gold pearls which are natural pearls with a layer of aragonite with an incredible lustre – so it does indeed look as if a layer of gold leaf has been applied to the pearls.
A large oyster (variety of Pinctada maxima) used in some countries to produce South Sea cultured pearls; it produces a yellowish nacre, and pearls that typically range from off-white to rich, deep gold in colour.
An optical phenomenon which causes the hue of the pearl to change colours depending upon the angle from which the pearl is viewed
A pearl which has only been partly drilled, as for rings or stud earrings. These sell for more than those which are fully drilled. The best have a flawless domed side.
The term hallmark is often confused with branding, but it is not a branding. Hallmarking is a specific process of assaying precious metals and marking them as having passed a required standard. It is a guarantee of quality of content. Hallmarking dates, in England, from 1300 when Edward 1 brought in a requirement for standardisation of silver for coin and wrought pieces, which were to be marked with the leopard’s head mark to show they had passed assay.
Variations on the leopard’s head are still used today for sterling silver pieces marked at the London assay office, run by the Worshipful Company of Goldsmiths. Different marks are used to denote fine silver, and the various grades of gold, plus platinum and palladium. A mark will show when and in which office the item was assayed, plus what metal and who made the item. The mark is either struck to the metal (the origin of ‘making one’s mark’ as the maker can optionally still strike their own sponsor’s mark before assay or the mark can be struck by laser. All Pearlescence precious metals over the required minimum weight carries the London Assay office hallmark of owner Wendy Graham (Initials wmg in an oval cartouche).
The triangle shell is the shell used to culture most freshwater pearls in China Irradiation. Irradiation has differing effects from freshwater to salt water cultured pearls. The gamma rays do not affect the nacre layers of a salt water cultured pearl, but in fact darken the nucleus of the pearl. An irradiated salt water pearl appears to be grey or blue. The nacre of freshwater irradiated pearls, on the other hand, if affected by the gamma rays and can become very dark. Some of these freshwater treated pearls will also have an intense metallic sheen and iridescent orient over their
These are a sub-species of freshwater pearls grown only in Lake Kasumi-ga-Ura, some 40 miles northeast of Tokyo, Japan. They have a distinctive surface, like wrinkled satin. Kasumi like pearls are now being produced in China
Keishi or Keshi
Japanese word meaning “something as tiny as you can imagine”, such as a grain of sand; used originally for very tiny gems that resulted by accident as part of the culturing process; now used to refer to all-nacre baroque pearls produced when something goes wrong in the process of culturing so that the seeding nucleus is ejected from the half formed pearl. South Sea keshi pearls can be very large; Japanese keshi pearls can be minuscule. The shape ranges
from resembling a cornflake (so they are also called cornflake pearls) to something more like a slightly deflated balloon. They tend to have fabulous lustre.
Knots in the silk between pearls is a sign of quality in pearls. If there are no knots or the pearls are on beading wire and look stiff and without movement then they are not being assembled to show their best. The knots serve two purposes. Firstly the chances of losing all the pearls is minimised, only one or two maximum can be lost (Pearlescence always gets really annoyed at the scene in ‘Murder is Announced’ where the pearl necklace breaks and all the pearls shower onto the floor. Good for Miss Marple but very bad for pearls).
Secondly each knot acts as a hinge allowing the necklace or bracelet to flex. They stop the individual pearls packing closely. The picture shows a two strand necklace where the upper strand has been strung unknotted onto silk and the lower has been knotted. Never get strung pearls wet – this is not because the pearls will be harmed, it is very unlikely that just getting wet with water (either salt, fresh or swimming pool) will damage pearls after all, but the silk on which they are strung will rot in time especially the silk inside each pearl which is trapped and therefore takes much longer to dry. Please do wear your pearls all the time. Pearls need light oils to look their best and the oil in human skin is perfect. If you absolutely must wear your pearls in water then please let us know and we can re-string them on nylon. Play between the pearl and the knots. This is a sign that the silk may be stretching and it might be time to start thinking about getting them re-strung. We are happy to re-string pearls and will restring our own pearls at a reduced rate.
A – AAA Grading system
Essence pearls are exceptional pearls, selected for highly metallic lustre, clean surface and shape, in that order. Usually only found by selecting in person. Probably under 1% of pearls will show the mirror metallic lustre we look for.
AAA: The highest-quality pearl, virtually flawless. The surface will have a very high lustre , not necessarily metallic, and at least 95% of the surface of each pearl is free of flaws. Any flaws are very small and hardly noticeable.
AA+ Nearly as good as AAA but perhaps slightly off round when rolled and a few more flaws although these will still only be visible on close inspection.
AA Average to good lustre, off round, blemishing to 20% of surface
A: This is the lowest jewellery-grade pearl, with a lower lustre and/or more than 25% of the surface showing defects. Probably a round pearl will be egg shaped, even from a distance Any website or other seller which talks about
AAAA+++ grade pearls is talking rubbish and this should be queried.
Tahitian pearls have a distinct and separate system, established by GIE Perles de Tahiti, and the Ministere de la Perliculture of Tahiti which grades from A (finest) to D ( poor) but to avoid confusion Miss Joaquim Pearls uses only the A to AAA gradings throughout the website.
Abalone blue pearls
Just being developed in New Zealand. The abalone produces a distinctive and stunningly iridescent blue pearl but is very hard to nucleate as its blood does not clot, so any damage will kill it.
A pearl from the akoya oyster (Pinctada Fucata Martensii). This is a salt water mollusc. Most cultured sea pearls are akoya pearls which are made with a bead nucleus, so that they usually have a good round shape. Big irregularities tend to be tails while less than perfect pearls have nacre with pits or convolutions. Good akoya pearls have a sharply reflective metallic lustre. Smaller (under 8mm) akoya pearls tend to come from China (although chinese production has dropped with the recession) while Japanese akoya pearl farmers are concentrating on producing larger high quality pearls (made-up necklaces marked Made In Japan may have been made with Chinese pearls if under 8mm) akoya pearls are harvested after only 9-16 months.
The problem is in obtaining pearls with sufficient nacre. Pearls with very thin nacre may even ‘blink’ which means that when rolled the nacre blinks to show patches where there is no nacre and you can see the nucleus. Below is a very bad example – the cream colour is nacre and the white is nucleus. Even when the nacre appears solid it can be very thin: peer closely and you can just about make out the thin line of the black nacre on the akoya pearl on the left (which split in half) The nacre on the pearl on the right is so thin the pearl is said to be blinking – if you roll it around it appears to blink, with sight of the nucleus.
Baroque pearls are strictly all non-round pearls but the term is usually applied to pearls which are not round but which nevertheless have a good rounded surface all over. Freshwater pearls are most commonly baroque as freshwater pearls are mantle-tissue nucleated instead of bead nucleated. So round pearls are the exception, although more are being produced as techniques improve. The most valuable baroque pearls are South Sea and Tahitian pearls which are produced by Blacklipped and White-lipped oysters (Pinctada margaritifera, and the Pinctada maxima). Commercial baroque pearls tend to be bigger pearls – there is a balancing act for the pearl farmer between leaving the pearl in the mollusc with the chance of a big round pearl and the likelihood that the pearl will go out of round and become baroque and therefore less valuable
All sea pearls are grown around a bead. It used to be that beads were not used in the production of most freshwater pearls (exceptions include coin pearls for example) However the last couple of years have seen the development of bead nucleation in freshwater pearls, producing second or third graft round pearls of stunning colour, lustre and shape. High quality bead nuked pearls are still exceptional and unusual and therefore very expensive, but can be up to 14mm. These freshwater pearls have been bead nucleated, and you can see the thick layer of nacre surrounding the nucleus
Or sometimes biwi-A freshwater pearl grown in lake Biwi in Japan. Not in the present as the pearl farms were closed due to pollution. Now often applied to any stick pearl
White pearls are colour treated by bleaching. This applies to both Akoya and Freshwater pearls. Black-lipped Oyster Pinctada margaritifera This oyster produces the Tahitian black pearl
Term to describe poor quality bead nucleated pearls where the nacre does not even fully cover the nucleus. When the strand is rolled the pearls look as if they are blinking.
A pearl that is attached to the inner surface of a mollusc shell
Often rounded on one side and flat on the other. Sometimes also called a fastener pearl . Most often used to make stud earrings, because in larger sizes round pearls can be too proud of the earlobe.
Classic term to identify the amount of gold in metal. Different metals are added to gold to harden it and make it more durable. Expressed as a fraction of 24 parts so that 24ct is fine gold or pure gold, down to the lowest standard which is 9ct in the UK, usually 14ct elsewhere.
No one knows exactly why some pearls develop circles. These can be bands of colour or grooves, as if the pearl has gently spun on its axis in the pearl sac. While circle pearls tend not to be the most expensive they are not as yet imitated and have stunning variety
Natural freshwater pearls tend to be shades of white through to pale pinks and peaches and golds The intensity of the colour depends on the species and strain of host mollusc plus the farm water and food. Tahitian and South sea pearls are not usually dyed.
Many pearls are coloured treated as part of the processing between farm and retailer. There is however, now a trend towards completely natural colour untreated pearls. Silver nitrate and gamma radiation are two treatments. (see separate entries) and white bleached akoya pearls are often ‘pinked’ – delicate tinted to a faint pink overtone which softens the colour and is supposedly more flattering and desirable.
Usually a round flat pearl shaped like a coin, also used to describe fancy hearts, squares, lozenge and other shaped pearls
Rarest of the natural pearls, conch pearls look a bit like jelly beans. They are not nacreous but have a distinctive flame pattern on the surface. The colours range from orange, through yellow to pink
Cook Island Pearls
Specific group of south sea islands which produce their own distinctive pearls from Pinctada Margaritifera. The pearls show the same colours as Tahitian pearls but are softer looking in shades
Very rare pearls produced by one farm in the Sea of Cortez, Mexico,from Concha Nácar, Pteria sterna, the rainbow lipped oyster. These pearls fluoresce red under UV light.
Cross can be diagonal or crucifix. Some cross pearls which also have nacre between the limbs have been sold as butterfly pearls
The cockscomb pearl mussel was the mollusc originally used by the Chinese when they started to culture freshwater pearls. The pearls produced are known as rice crispie pearls because of their resemblance to the cereal
A pearl formed after a human puts a bead nucleus or mantle tissue into a mollusc. Any farmed pearl is cultured. Any real pearl feels faintly gritty when rubbed gently on your teeth and the drill hole tends to be very small (usually 0.7mm)(because pearls are still often sold by weight)
Culturing Freshwater Pearls
In freshwater mussels, insertion of only mantle tissue is enough to trigger the making of a pearl sac and therefore pearl production. It used to be that beads were not used. However the last couple of years have seen the development of bead nucleation in freshwater pearls, producing second or third graft round pearls of stunning colour, lustre and shape. High quality bead nuked pearls are still exceptional and unusual and therefore very expensive, but can be up to 14mm. Even larger pearls are being produced with pearls nucleated with a lump of mud (!) these pearls, third graft, are of stunning lustre and a rather keishi appearance so far. When drilled the mud is drained away so that
the pearl is hollow and light in weight. However most freshwater cultured pearls are still solid pearl nacre, even pearls up to 15mm. This means that they are arguably more durable but the chances of non-perfect round shapes are higher
Since so many good quality white fake pearls are now available the trend is for natural colour pearls to remain untreated. Usually white freshwater pearls have to be bleached. There are many natural colours of freshwater pearls; pink, peach, purple, yellow, white, grey, brown, champagne and black. Only freshwater pearls are ever pink, peach and purple.. Black pearls are created by black oysters. The darker the colour is, the more valuable the pearl and black pearls with a little bit of green are the most precious.
Other colours are created artificially by dyeing or irradiating the pearls, or treating chemically. It is quite hard to tell with some colours whether or not a pearl has been treated (although a deep blue or hot pink pearl is never natural). Irradiated pearls are often silver/grey, blue, green, or gold to brown. Most dyed pearls are colourfast, and irradiated pearls won’t lose their colour, and are not radioactive.
While Salt water oysters will only manage to make one pearl each (which keeps up their scarcity and value) freshwater mussels are more obliging and will make 20 or more each. Some farms are developing their own strains of mussel, selecting for quality, while other farms will buy in their mussels ready nucleated. This careful breeding is producing more strongly coloured natural colour pearls. After harvest in China pearls go from individual farms to pearl factories where they are bleached to be white pearls, or otherwise coloured or processed, drilled and sorted, and assembled into strands.
Culturing Saltwater Pearls
Several distinct types of pearls grow in salt waters. Farming methods are pretty much the same for all of them
The process of growing sea pearls in oysters was discovered (or re-discovered as there are arguments about this) by Mikimoto in 1893. All pearls which grow in salt water start with baby oysters which are either artificially bred in a
hatchery or spawn naturally then are collected by placing various lures in the water to attract the spats as they are called. The baby oysters are grown on for two or more years until they are big enough to manage to accept a grafted bead nucleus.
With all sea pearls the pearl is grown around a nucleus – a starter bead plus a tiny fragment of mantle tissue which grows to form a pearl sac around the bead. As the mantle tissue is tissue for making nacre/shell it carries on doing this, secreting nacre on the inside of the sac and onto the bead. Mantle tissue makes the pearl sac because its job normally is to secrete the mother of pearl to make the smooth and lustrous lining of the oyster’s shell.
Early in the morning of the day an oyster will receive a nucleus, it is taken out of the water and then left for about half an hour, by which time it should have opened its shell a little. The shells are wedged open. Any unopen shells go back into the water to be left for another attempt in a few days Nucleating oysters is a skilled task – even opening the shell too far can kill the delicate creature. The bead-plus-mantle tissue scrap is inserted into an incision into the body of the oyster, either at its gonad or by the connective tissue. Remarkably having a bead stuck into its sex organ seems to make the oyster more active sexually rather than less!
A nucleus is a (usually) round bead made from shell and cut and polished into a smooth round -usually about 8mm in diameter for first grafting The oyster is secured in a clamping device in front of the operator and either the wooden wedge is left in place or a retractor which allows the shells to be forced further apart is inserted. If the oyster is opened too far it will die. The aim is for this process to take under a minute and it is reckoned that it takes a month at least for the oyster to recover.
The actual process is that the grafter, working through the tiny opening between the two halves of the shell, makes n incision of about a centimeter into the oyster’s gonad or into its connective tissue then places the mantle tissue and nucleus (dipped in water and held by a suction tool) into this slit. The two insertions must be touching, or a pearl sac will not form. Then the oyster is put back into the sea. There are various ways it is held but they all work to allow the oyster to feed happily and grow. No-one knows exactly why some grafts become great pearls and others don’t. It is probably a mixture and combination of genetics, grafting skill, and growing conditions. Many farms keep a record to see who is the best grafter (!)
The implanted tissue forms a pearl sac around the nucleus and starts to secrete nacre. It will take between two and four years for the pearls to form. The tissue implant is only about 1mm square. It will form the pearl, which has no genetic relationship with the host mollusc. Nacre is mostly carbonated calcium. As long as the irritant is present the mollusc continues to add layers of nacre until a smooth lustrous pearl is formed.
Only one pearl per oyster can be produced. Sometimes oysters can be re-nucleated after harvesting to produce a bigger pearl with a bigger nucleating bead, or, if no bead is used a keishi pearl can be produced (think of the inside of an inflated then deflated balloon) Oysters are fairly fussy about their conditions and if forced to open too much they will die, as they will if they are out of the water too long, get too hot or too cold, if the water in which they live becomes too saline or not saline enough (this happens when a river floods and any oysters living in the estuary may well die because of the temporary dilution of salinity. It takes about 18 months to two years to grow tahitian and south sea pearls. Tahitian pearls are required by local law to be x-rayed and have a minimum nacre depth of 0.8mm all round. South sea pearls tend to have much thicker nacre than this.
There is some controversy about how long akoya pearls need to stay in the water. Some are harvested after only six months but these pearls can have gaps in their nacre so the bead is visible (they are said to ‘blink’ when rolled) and they will wear out quickly. But they will, of course, be very much cheaper. The pearls are cosseted. They will be cleaned several times to remove algae, vegetable growths and barnacles, and the farmer must keep an eye on the weather conditions – some akoya farms now monitor temperature and salinity and move the oysters if conditions are not ideal.
IDENTIFICATION OF IRRADIATED SOUTH SEA CULTURED PEARLS USING ELECTRON SPIN RESONANCE SPECTROSCOPY
Irradiated South Sea cultured pearls (SSCPs) from the Pinctada maxima mollusk typically show colors from light gray to silver. It is difficult to identify gamma-ray irradiation of SSCPs using standard gemological methods because of their thick nacre. Therefore, an advanced analytical technique such as electron spin resonance (ESR) spectroscopy is needed to detect the treatment.
ESR measurements of minute amounts of SSCP powders revealed the formation of CO2 – radicals, and the parameter known as the g-factor was
measured at 2.0015 ± 0.0005. Higher levels of CO2 – radicals were detected in the pearl nacre than in the nucleus. Therefore, the existence of CO2 – radicals is an indicator of irradiated SSCPs.
Irradiation, dyeing, bleaching, and heat treatment are widely used methods to alter pearl color. Although most artificial colors are easily recognized, some resemble attractive colors that occur in nature (Elen, 2001; Li and Chen, 2001; Zachovay, 2005; Wang et al., 2006; “Better techniques improve brown pearls,” 2006; McClure et al., 2010).
Lower-quality freshwater and saltwater cultured pearls are regularly exposed to 60Co gamma-ray radiation in an attempt to simulate black pearls or enhance orient (Crowningshield, 1988; Li and Chen, 2002; O’Donoghue, 2006). In recent years, the irradiation process has been applied to not only Akoya cultured pearls and freshwater cultured pearls (FWCPs), but also to South Sea cultured pearls (SSCPs) (Choi et al., 2012). The irradiation-induced color change results from the darkening of the nucleus, caused by MnCO3 oxidation, as well as denatured damage to the
pearl’s conchiolin (Matsuda and Miyoshi, 1988). FWCPs have a higher abundance of proteinous components and manganese than saltwater pearls (Hatano and Ganno, 1962).
Gamma-ray irradiated SSCPs (figure 1) were first discovered in the Korean market in April 2011. At the March 2011 Hong Kong Jewelry Show, a Japanese trader reportedly sold a Korean counterpart irradiated SSCPs without disclosing the treatment. They were light gray or silver loose cultured pearls and beads 10–16 mm in size. While a cream, yellow, or black color is produced by a protein pigment in the nacre, a blue or silver color is caused by organic material between the nacre and nucleus (Komatsu, 1999; O’- Donoghue, 2006). Korean consumers typically prefer SSCPs with a silver color created by organic material. According to the research of Choi et al. (2012), gamma-ray irradiated SSCPs with colors ranging from white to cream turned light gray to silver, with the depth of color correlating with increasing irradiation dose. A dose of 0.5–1 kGy caused a light gray color, while a dose above 5 kGy produced a silver color.
For Akoya cultured pearls, with a typical nacre thickness of 0.2–0.6 mm, irradiation can be identified through standard gemological tests (Komatsu, 1999; O’Donoghue, 2006). But for SSCPs, which have a nacre thickness of roughly 1.5–3.0 mm, detecting irradiation is difficult with methods such as transmitted light, magnification, fluorescence reaction, and UV-Vis spectrometry (Choi et al., 2012). This study attempted to identify irradiated SSCPs using electron spin resonance (ESR) spectroscopy. This method, also known as electron paramagnetic resonance (EPR) spectroscopy, identifies the presence of unpaired electrons. Moreover, the study sought to minimize damage during examination by obtaining a minimal sample of powder from each cultured pearl.
MATERIALS AND METHODS
For the study, some 300 SSCPs weighing 6.55–18.05 ct (8.0–16.6 mm in diameter) with white to cream color were exposed to gamma-ray irradiation at room temperature. The irradiation was conducted at the 60Co facility of the KAERI (Korea Atomic Energy Research Institute) in Jeongeup, South Korea. The absorbed doses were set at 0.2, 0.4, 0.6, 0.8, 1, 5, and 100 kGy.
Inductively Coupled Plasma-Atomic Emission Spectrometer. Chemical composition analyses of the SSCPs were performed with an inductively coupled plasma–atomic emission spectrometer (ICP-AES, Varian Vista-PRO). The nacre, nucleus (bead), and conchiolin were separated and powdered, and 0.2 g of each powder was dissolved in a solution of 37%HCl (6 ml) and 65% HNO3 (2 ml). We tested the samples after 20 minutes at 200°C and after 10 minutes at the same temperature to obtain an average value. Electron Spin Resonance Spectroscopy. This study relied on electron spin resonance analysis to observe radicals produced by the irradiation process. The ESR spectrometer gauges the absorbed dose corresponding to the splitting energy of unpaired electrons in a magnetic
field. The technique can rapidly identify an irradiation- related signal from a small amount of sample in a few minutes. For this study, we collected at least 10 mg of SSCP powder from both the nacre and the nuclei of each cultured pearl. To determine if the ESR signals correlated with Mn2+, solid samples of FWCP, which contain more manganese than SSCPs, were irradiated with a 100 kGy dose.
Room-temperature ESR spectra were recorded using a JEOL FA-300 spectrometer with a manganese marker (MgO: Mn2+), using 9.8 GHz microwave frequency, 1 mW microwave power, a 1–2 G modulation amplitude, a 2 min sweep time, and a 0.03 s response time (figure 2).
Mn marker for ESR analysis. The g-factors of free radicals created by irradiation are approximately 2.00. For comparison, the “free electron” g-factor is 2.0023. Standard reference samples can be used to correct for any systematic errors in the measured magnetic field values and to verify the sensitivity of the system. Standard samples include DPPH (2.2-diphenyl-1-picryl-hydrazyl), TCNQ-Li (tetracyanoquino-dimethane Li saly), CaO:Mn2+, and MgO:Mn2+. The choice of standard sample used depends on what the user wants to determine. For example, DPPH is used to calculate gfactors, to monitor the sensitivity of the equipment, and to quantify spin concentrations. TCNQ-Li is used to find the g-factor. CaO:Mn2+, MgO:Mn2+, and Mn2+ are used to measure the g-factor and to correct magnetic field variations.
The g-factor of most standard samples is also located around 2.00. The Mn marker is shown with six Mn2+ signals; the third (2.034) and fourth (1.981) signals are used to correct magnetic field variations. Each signal has a regular interval from 2.00. From this property, the MgO:Mn2+ marker could be more suitable to measuring the g-factor than the alternative standard samples. The MgO:Mn2+ marker was supplied with the Jeol X-band spectrometer in the shape of a small rod that can be electromechanically inserted externally into the microwave cavity. When a sample and a Mn marker are measured simultaneously, the resulting ESR spectrum will contain signal contributions from both. It is easy to distinguish the ESR spectra of one from the other, since the Mn2+ signals have the opposite phase to that of the sample’s signal (i.e., the signal’s lineshape will appear to have been flipped across the baseline). RESULTS AND DISCUSSION
The major element of a pearl is calcium. Chemical composition analysis of bead-cultured pearls using ICP-AES demonstrates that the nacre and the freshwater nucleus contain similar trace elements but vary in their composition. The nacre contains more Na, Mg, and Sr, while the nucleus has higher Mn and P contents (table 1).
After 60Co gamma-ray irradiation at a dose of 5 kGy, the SSCPs exhibited gray to silver coloration (figure 3). The interior of one of the irradiated pearls revealed a grayish brown to dark gray nucleus, along with an altered nacre color (figure 4). The irradiationinduced color change is chiefly attributed to the darkening of the nucleus (bead), which in turn darkens the nacre—especially in the thinner-skinned Akoya cultured pearls (Komatsu, 1999). As shown in this experiment, color change took place in the nacre as well.
Figure 5 shows that the concentration of radicals produced by irradiation exposure increases with the absorbed dose. Formerly undetected free radicals were observed after a low-dose radiation of 0.2 kGy. The g-factor was 2.0015 ± 0.0005, which agrees with that of CO2 – radicals (Wieser et al., 1985; Ikeya, 1993; Seletchi and Duliu, 2007). With higher absorbed doses, the CO2 – radical signal intensity further intensified. The identification of CO2 – radicals through ESR analysis thus serves as a way to distinguish irradiated cultured pearls.
Matsuda and Miyoshi (1988) reported that the irradiation – induced change of color is caused by manganese (Mn). They noted that MnCO3 in the nucleus (bead) turned into oxidations such as Mn3O4, Mn2O3, and Mn2O after irradiation. Their results are still cited in literature related to color change in irradiated pearls (e.g., Komatsu, 1999; Wada, 1999; McClure, 2010).
Yet existing mechanisms are insufficient to explain the alteration of pearl color by irradiation (Li and Chen, 2002). Based on the results of gamma-ray irradiation tests in this study, the authors believe that post-irradiation color change cannot solely be attributed to MnCO3 oxidation. Two factors support this hypothesis:
After irradiation, the pearl nacre blackened to a similar extent as the nucleus (bead), even though it contains approximately 20 times less Mn (see figure 4 and table 1). Figure 6 is an ESR spectrum comparing untreated FWCP, irradiated (100 kGy) FWCP, and a Mn marker (MgO: Mn2+) attached to the JEOL equipment. The Mn marker consists of Mn2+ and shows six sharp peaks in the ESR spectrum (figure 6b). Before (figure 6a) and after (figure 6c) irradiation spectra of FWCPs (typical in the carbonate spectrum) do not match the positions of the Mn2+ signals. Nevertheless, a change was observed in the spectra before and after irradiation: the formation of CO2 – radicals between the third and fourth Mn2+ peaks (highlighted by the green circle in figure 6c). Because these results were the same among all SSCPs investigated in this study, peaks in the ESR spectrum are unrelated to Mn.2.CO2 – radicals appeared as irradiation doses increased and multiplied in proportion to the dose (figure 7). The intensity of CO2 – radicals was also proportional to the blackening of the pearl nucleus (bead). The CO3
molecular ion in CaCO3 is easily ionized by radiation. Elementary defects induced by ionizing radiation are an electron center ( CO3- 3–) and a hole center (CO3–). While the CO3- 3– and CO3 – centers are stable at low temperatures, the electron center CO2 –, formed by irradiation, is an electron center similar but more stable than CO3- 3– (Ikeya, 1993). Additionally, we found that the color of nacre and nucleus had been bleached
under incandescent light (approximately 50°C) for 30 days. The color changed by irradiation and heat (by light) is related to the color center. Therefore, the color change of the nacre and the blackening of the nucleus (bead) are believed to be related to color centers formed by CO2 – radicals. Choi et al. (2012) found that after irradiation, glutamic acid decreased 11.43% (from 3.5% to 3.1%), alanin 21.8% (from 22.5% to 21.8%), and histidine 43.75% (from 1.6% to 0.9%), according to amino acid analysis to examine the change of protein between aragonite platelets in pearl nacre. Hatano and Ganno (1962) found that gamma-ray irradiation destroyed 32% of the histidine, 16.6% of the methionine, 11% of the glutamic acid, and 9.3% of the proline in the
protein of the FWCPs. The destruction of protein caused by irradiation can also alter the color of SSCPs.
CO2 – radicals at the absorbed irradiation dose of 0.2 kGy are barely visible in the nucleus sample but far more intense at doses above 0.4 kGy (figure 5, right). In particular, CO2 – radicals emerging after irradiation were better observed in the nacre than in the nucleus at the same absorbed dose (figure 5, left).
After normalizing the results of figure 5 to a nonirradiated spectrum (0 kGy, black line), the increased intensity of radicals was calculated by peak-to-peak height. The intensity of the CO2 – radical is stronger in the nacre than in the nucleus when irradiated with a dose above 0.4 kGy (figure 7).
Ikeya (1993) reported that Mg2+ ions might be accompanied by H2O molecules, leading to a rapid reduction in hydrated radicals. The saturation level of isotropic CO2 – also increases with the Mg/Ca ratio. Barabas et al. (1989) studied synthetic carbonate crystals doped with Mg2+ and observed the following:
ESR spectra that displayed signals at the same spectroscopic properties as natural carbonates; and
an increase of the g-factor signal with Mg concentration in the carbonate crystals. Mg also plays an important role in the formation of the crystal lattice of carbonates (Katz, 1973) and may enhance the formation of specific defects (Barabas et al., 1992). Lattice distortions caused by the incorporation of Mg2+ ions (Goldsmith and Graf,1958) may lead to CO2 – by creating larger interatomic distances (Barabas et al., 1992). In this context, thehigher abundance of CO2 – radicals in the nacre is thought to be related to the Mg/Ca ratio.
Considering the combined published observations on Mg2+ and CO2 – (Ikeya, 1993; Barabas et al., 1989, 1992; and Katz, 1973) it is likely that the saturation level of CO2 – rises proportionally with the Mg/Ca ratio in pearls of this study. As shown in table 1, the nacre and the nucleus (bead) contain 100 and 26 ppm of Mg, respectively. The nacre’s Mg/Ca ratio is approximately four times greater than that of the nucleus (bead). Mg, which is more abundant in the
nacre, therefore results in the preferential formation of CO2 – in the nacre rather than in the nucleus when exposed to the same absorbed radiation dose. This is consistent with the higher CO2 – ESR signal intensity observed in the nacre than in the nucleus (again, see figure 5). This suggests it is possible to identify an irradiated SSCP using ESR spectroscopy.
Identifying irradiated SSCPs through traditional gemological methods has been difficult, as their nacre is usually quite thick. But as this ESR study demonstrates, the separation of untreated pearls from irradiated pearls is possible. In doing so, an infinitesimal amount of sample was taken from the nacre in the form of powder. After irradiation, CO2
– radicals were formed, and their presence was confirmed using ESR spectroscopy. The amount of CO2 – radicals increased in proportion to the irradiation dose, and they were more observable in the nacre than in the nucleus
(bead). Until now, irradiation-induced color changes in pearls were thought to be due to the change of the MnCO3 oxidation number. But as this study notes, such color alteration is apparently related to an alteration caused by protein destruction rather than Mn, as well as color centers created by CO2 – radicals.
Gamma-ray irradiation is routinely applied to South Sea cultured pearls (SSCPs), typically producing a light gray to silver color.
For SSCPs, which have a particularly thick nacre, detecting irradiation is difficult using methods such as transmitted light, magnification, fluorescence reaction, and UV-Vis spectrometry.
Electron spin resonance (ESR) spectroscopy rapidly identifies the presence of CO2 – radicals, whose concentration is proportional to the absorbed irradiation dose.
Articles source: Youngchool Kim, Hyunmin Choi, Bohyun Lee, and Ahmadjan Abduriyim – GEMS & GEMOLOGY, WINTER 2012
Wholesale pearls for jewelry making : UPDATE ON THE IDENTIFICATION OF DYE TREATMENT IN YELLOW OR “GOLDEN” CULTURED PEARLS
Dye treatments in yellow or “golden” cultured pearls have improved to the point that some samples show little surface evidence. In addition to routine gemological observations, analytical techniques such as UV-Vis reflectance and Raman photoluminescence (PL) spectroscopy are critical to identifying the treatment. This study demonstrated three indications of dye treatment: broad reflectance features between 410 and 450 nm, the lack of a reflectance feature at 350 nm in the UV-Vis spectra, and intense fluorescence in the visible spectrum under 514 nm wavelength laser excitation. These diagnostic features may be used independently, even when no visual evidence of a dye exists. (refference : wholesale pearls for jewelry making )
(refference : wholesale pearls for jewelry making )
Most dyed yellow or “golden” cultured pearls can be identified with routine microscopic observations. Dye residues usually accumulate within drill holes and surface blemishes, making them easy to detect with magnification. In some cases, long-wave UV fluorescence and UV-Vis reflectance spectrophotometry have been used to provide further evidence of dyeing (Elen, 2002; Qi et al., 2008; Chen et al., 2009).(refference : wholesale pearls beads )
In recent years, though, GIA has begun receiving more “golden” cultured pearls with a typical UV fluorescence or UV-Vis reflectance characteristics but no evidence of dye residue. As processing techniques continue to improve, the authors believe it is important to update the trade on the situation to make sure that current identification methods are up to par with the treatments. The term “golden” is used to describe mid- to light-tone cultured pearls with a strong saturation in the yellow and orangy yellow hues (Gemological Institute of America, 2000).(refference : wholesale pearls beads )
These cultured pearls are formed within Pinctada maxima (gold-lipped) oysters and have gained popularity over the years with the help of extensive marketing efforts by the industry (Shor, 2007; “The fabulous golden pearls of the Philippines…,” 2010). In the meantime, increasing amounts of dyed “golden” South Sea and freshwater cultured pearls (“Supplier warns trade against dyed golden,” 1998; Roskin, 2005) and, to a lesser extent, heat-treated “golden” products have also appeared on the market (Elen, 2001 and 2002).(refference : wholesale pearls beads )
Detecting the treatment remains an important consideration in pearl identification, and an ongoing research investigation at GIA aims to provide solutions to the issue. The present study focuses on the identification of eight sample groups of yellow or “golden” cultured pearl using routine gemological testing methods and advanced analytical techniques (figure 1). (refference : wholesale pearls beads )
(refference : wholesale pearls for jewelry making )
The known dyed samples exhibiting no traces of surface dye concentrations were singled out for analytical testing. The results suggest that advanced techniques such as UV-Vis reflectance and PL spectroscopy can detect the dye even when surface concentrations are absent. More than 100 naturally colored yellow cultured pearls were tested with the UV-Vis reflectance technique to provide additional reference datasets. Some of the latter were also tested with PL for the same purpose. Lastly, three heat-treated yellow cultured pearls were tested; their results are discussed briefly, since the sample size is minimal. (refference : wholesale pearls beads )
MATERIALS AND METHODS (refference : wholesale pearls for jewelry making )
A total of 69 yellow and “golden” cultured pearls ranging from 6.5 to 14 mm were studied. The eight sample groups consisted of:
10 naturally colored South Sea cultured pearls from the Philippines (NSSP)
8 naturally colored South Sea cultured pearls from Myanmar (NSSM)
21 dyed South Sea cultured pearls, in three separate groups (DSS, DSS2, and DSS3)
20 dyed akoya cultured pearls, in two separate groups (DAK and DAK2)
10 dyed freshwater nonbead-cultured pearls (DFW)
These samples were obtained from reliable sources who provided information on the samples’ provenance. Real-time micro-radiography examination with a Faxitron CS-100-AC confirmed they were all cultured pearl products. Each sample was examined with a standard gemological microscope, and photomicrographs were taken using a Nikon SMZ 1500 stereo-microscope. Fluorescence reactions were observed in a darkened room using a conventional 5-watt long-wave (366 nm) UV lamp. UV-Vis reflectance spectra were obtained using a Perkin Elmer Lambda 950 UV-Vis spectrophotometer with an integrated sphere accessory. (refference : wholesale pearls beads )
Selected samples from each group were also tested with a Thermo Nicolet Nexus 670 FTIR spectrometer and a Renishaw inVia Raman microscope. The three heat-treated cultured pearls were obtained from a reliable source. In addition, more than 100 naturally colored yellow or “golden” South Sea cultured pearls (from Jewelmer) were tested using an Ocean Optics USB 2000+ UV-Vis spectrometer. This unit takes less than one minute to run a pearl sample, making it ideal for rapidly examining bulk quantities. Some of these cultured pearls were also tested with PL spectroscopy. A summary of the various sample groups and advanced testing techniques is provided in table 1. (refference : wholesale pearls beads )
RESULTS (refference : wholesale pearls for jewelry making )
Gemological Observations and UV Fluorescence. All cultured pearls exhibited light yellow, orangy yellow, yellow, or strong yellow bodycolors of uniform color distribution except the dyed samples from group DSS3, which showed distinctly uneven color distribution. Under magnification, concentrated dye features were observed in three additional dyed groups (DSS2, DAK2, and DFW), while the other two dyed groups (DSS and DAK) showed no evidence of surface treatment (figure 2). (refference : wholesale pearls beads )
(refference : wholesale pearls for jewelry making )
To make matters even more challenging, cultured pearls from the DSS group did not possess drill holes, which serve to enhance the diffusion of the dye material, suggesting that a different dyeing technique was applied to them. Representative samples from groups NSSM, DSS, DSS2, and DAK were cut in half to observe the color distribution throughout their cross-sections.(refference : wholesale pearls beads )
Typical concentric growth rings were noted on the nacre of the naturally colored sample, while the growth structures in the dyed cultured pearls were largely masked by the infiltration of dyes. The presence of a drill hole in the samples from DSS2 and DAK had caused the dye materials to diffuse into the bead used to culture the pearls. UV fluorescence generally followed the body-color of the sample. Naturally colored orangy yellow to strong yellow cultured pearls usually exhibited weak yellow fluorescence, while lighter yellow samples exhibited moderate to strong yellow fluorescence.(refference : wholesale pearls beads )
It is a challenging task, however, to accurately and consistently describe fluorescence color, since there is no reference for comparison. In this study, dyed samples also showed varying degrees of yellow or orangy yellow fluorescence, but not distinctive enough to consistently separate them from the naturally colored variety. Samples from DSS3 and DFW showed uneven color distribution due to dye concentrations on their surfaces. General observations and measurements are shown in table 2.(refference : wholesale pearls beads )
UV-Vis Reflectance Spectra. Within each group, UVVis reflectance properties were generally consistent. Naturally colored samples (NSSP and NSSM) showed decreasing reflectance toward the lower visible and long-wave UV range, with subtle local reflectance troughs at about 350 and 440 nm (figure 3). These reflectance troughs may be due to (but not equal to) absorptions at specific wavelengths. Cultured pearls from five of the dyed groups (DSS, DSS2, DSS3, DAK, and DAK2) all showed distinct reflectance characteristics within the same range, but with broader, more prominent, and sometimes shifted reflectance features between 410 and 450 nm, consistent with previous findings (Elen, 2002; Qi et al., 2008; Chen et al.,2009).(refference : wholesale pearls beads )
Some of the dyed cultured pearls (DSS, DAK, and DAK2) also lacked the 350 nm reflectance feature, while others (DSS2 and DSS3) showed a steeper slope between 430 and 480 nm than that of naturally colored samples (figure 4), also consistent with previous findings. Dyed freshwater cultured pearls showed reflectance patterns similar to those of the naturally colored samples within the lower visible range, but lacked the 350 nm reflectance feature. FTIR, Raman, and PL Spectroscopy Results. We performed infrared and Raman spectroscopy on representative samples from each of the eight groups. The FTIR spectra only showed the vibrational modes of aragonite, the major component of all pearls, dyed or naturally colored (figure 5). Raman spectroscopy was performed with both 514 and 830 nm lasers. The 830 nm laser gave much better peak resolution while the 514 nm laser (data not shown) registered significantly higher background fluorescence in the dyed and naturally colored samples.(refference : wholesale pearls beads )
To clearly visualize the fluorescence characteristics of these samples upon laser excitation, we performed PL measurements. These confirmed that most of the dyed cultured pearls fluoresced at much higher levels than naturally colored pearls—in a few cases, reduced power had to be used to prevent peak oversaturation—making it a useful tool in identifying some cases of dye treatment (figure 7). A more useful way to look at the data, though, is to compare the ratio between overall fluorescence intensity (600–700 nm) and the height of the main aragonite peak at 545 nm (i.e., the F/A ratio; figure 8). Dominant or significant aragonite peak intensities were observed in the spectra of naturally colored samples, with the F/A ratio consistently below 5. For dyed samples, the ratio varied more due to the different dye materials used, but they were more likely to have F/A ratios of at least 10.(refference : wholesale pearls beads )
Additional Reference Collection Data Results. In addition to the 18 reportedly naturally colored yellow samples, we examined more than 100 reportedly naturally colored yellow to orangy yellow cultured pearls of various saturations using UV-Vis reflectance and PL methods. These provided useful baselines for comparing unknown samples. The UV-Vis reflectance results of these naturally colored yellow samples showed consistent spectroscopic characteristics, similar to those observed in groups NSSP and NSSM (again, see figure 3). Low PL fluorescence signals (and F/A ratio) were also observed in all of the cultured pearls. Building and maintaining a spectral database from naturally colored yellow samples of various saturations (figure 9) is important for comparative analysis and identification of dye treatment.(refference : wholesale pearls beads )
(refference : wholesale pearls for jewelry making )
Heat-Treated Yellow Cultured Pearls. In addition to dye treatment, heat-treated yellow cultured pearls have been reported (Elen, 2001). The exact mechanism of color alteration is still unclear. One theory suggests that heating changes the amino acid compositions of conchiolin proteins, altering their physical and chemical properties (Akiyama, 1978). Another possibility is that heating proteins and sugars (found in conchiolin) at high temperature under intermediate moisture levels and alkaline conditions will promote Maillard reaction, resulting in a color change similar to the browning effect caused by heating many kinds of food.(refference : wholesale pearls beads )
The three reportedly heat-treated cultured pearls were tested using UV-Vis reflectance and PL spectroscopy. The UV-Vis spectra lacked the obvious broad reflectance pattern found in dyed samples, consistent with an earlier report (Elen, 2001) that their heat treatment did not involve any addition of dye materials. Yet the PL spectra showed extremely intense fluorescence, which could be useful in separating them from naturally colored samples. A brief summary of these results appears in box A.(refference : wholesale pearls beads )
DISCUSSION (refference : wholesale pearls for jewelry making )
The dyeing of cultured pearls has been a common practice for many years (Alexander, 1960; Liddicoat, 1962; Johnson and Koivula, 1999), and it can usually be detected through careful examination of the surface. In our study, four of the six groups of dyed yellow or “golden” samples could be detected through conventional microscopic observation. Concentrated dye residues and uneven color distribution provided definitive evidence. These products are usually treated after drilling, which was confirmed by the dye residue within and around the drill holes. The other two groups (DSS and DAK) had relatively clean surfaces, and even a trained gemologist would have difficulty in separating them from naturally colored samples. Cultured pearls from the DSS group were treated without the aid of drill holes, while samples from the DAK group were dyed either before or after drilling. If they were dyed after drilling, further treatment such as bleaching may have been used to lighten any color concentrations that accumulated near the drill holes.(refference : wholesale pearls beads )
All the yellow or “golden” cultured pearls showed decreasing reflectance in the violet/blue region of the visible spectrum, which corresponds with the color reflected, in accordance with complementary color theory and human color perception. But naturally colored samples displayed a gradual decrease in reflectance, with subtle local reflectance troughs at showed significant reflectance troughs between 410 and 450 nm. These distinct reflectance characteristics can be explained by the different reflectance properties of natural pigments and the predominantly single-component artificial dyes applied to the treated products, as well as the variable concentrations of either.(refference : wholesale pearls beads )
Interestingly, the origin of the golden color found in South Sea cultured pearls may also be derived from nano-composite structures of the nacre, as reported by Snow (2004), which helps further explain the different reflectance features between naturally colored and dyed cultured pearls. For the DFW group, no significant differences were found in the violet/blue region of the visible spectrum, in part because they contained less dye than the other groups. Some of the dyed groups also lacked the local reflectance trough at 350 nm, which occurs almost exclusively in cultured pearls with yellowish hues and may be attributed to a particular pigment. Although some previous studies have reported the presence of natural pigments in naturally colored freshwater, Tahitian, and Pteria species samples (Karampelas et al., 2007; Bersani and Lottici, 2010), our study found no obvious differences using either infrared or Raman spectroscopy. Low pigment or dye concentrations, the location of these materials inside nacre platelets, and strong signal interference by aragonite crystal structure of the pearls could all make it difficult to detect any pigment or dyes using Raman spectroscopy. Yet dyed cultured pearls generally show higher PL under 514 nm laser excitation, likely a consequence of the fluorescence characteristics of the particular dye(s) applied. The result agrees with earlier studies (Liu and Li, 2007; Chen et al., 2009).(refference : wholesale pearls beads )
CONCLUSIONS (refference : wholesale pearls for jewelry making )
While most dyed yellow or “golden” cultured pearls can still be detected with relative ease using magnification, some show very clean surfaces lacking any evidence of dye. We have demonstrated that these can be identified by nondestructive, advanced instrumental techniques such as UV-Vis reflectance and PL spectroscopy. Our study suggests three indications of dyeing: broad reflectance troughs between 410 and 450 nm, a lack of a reflectance feature at 350 nm, or intense fluorescence in the visible spectrum under 514 nm wavelength laser excitation. When testing cultured pearls using advanced instrumentation, comparative analysis between naturally colored and dyed samples is an important part of the identification process in certain cases.(refference : wholesale pearls beads )
GIA has collected sets of data from numerous naturally colored yellow or “golden” cultured pearls with varying degrees of saturation to use as references for comparison against the spectra of unknown samples. Further analysis of “golden” cultured pearls is needed due to the unlimited number of dye materials that can be used to treat off-color or low-grade goods.(refference : wholesale pearls beads )
Articles source: Chunhui Zhou, Artitaya Homkrajae, Joyce Wing Yan Ho, Akira Hyatt, and Nicholas Sturman – GEMS & GEMOLOGY, WINTER 2012 (refference : wholesale pearls beads )
COLLECTION AND EXAMINATION OF PEARLS FROM WILD AND HATCHERY SHELL
Whenever natural nacreous pearls are spoken of, the tendency is to think of pearls from the Gulf region, which are produced mainly by Pinctada radiata. Indeed, one young European dealer was overheard saying that the only natural pearls are “Basra” pearls. Many are surprised to discover that high-quality natural pearls are also being produced by Pinctada maxima or at all. Hopefully this paper will serve to address trade misconceptions.
Recently, questions have been raised in some gem laboratories concerning nacreous pearls from Pinctada maxima. These questions are related to the difficulty in some instances of determining whether a pearl from this mollusk is natural, non-bead cultured, or even bead-cultured using a natural or non-bead cultured (atypical) bead. Indeed, some labora tories may have taken, for a time, the extreme measure of not issuing identification reports on any nacreous pearls from Pinctada maxima. An understanding of the Pinctada maxima has therefore become vital to the health of the natural pearl trade; the alternative is for the pearl business to become relevant only to the antiques market, with questions hanging even over these. Further, as the Pinctada radiata mollusk begins to be used in the Gulf for pearl culture, so too will the same questions need to be addressed with regard to this mollusk.
MATERIALS AND METHODS
Assuring sample integrity has always been a challenge within the gemological community. For the most part, gemologists have proceeded with research based on samples that have been donated or loaned rather than attempting to secure a higher degree of reliability concerning their origin. With gemstones, the highest degree of integrity is assured when a member of the research team collects samples in situ at the mine site, records the find/extraction in precise detail, and secures these samples in such a manner as to avoid any contamination. With pearling, the challenges are often at least equal. We addressed sample integrity by first observing the thoroughness of Paspaley’s stock control systems for both wild and hatchery shell and then working with them in a spirit of complete openness. Over several years, as wild shell were fished and “relaxed” aboard the vessel, the mantle in the area of the opening was inspected for likely natural pearls prior to putting them on the production line. The authors asked that video be taken of any pearls found still in the mantle of these wild shells. As more were eventually discovered, we were invited onboard to record them ourselves and retrieve the pearls and shell for examination in the laboratory. Between July 26 and 29, 2011, the authors achieved their goal and left Western Australia with a clear understanding of how natural pearls are discovered within P. maxima shell, along with a small but suitable group of samples for laboratory examination (table 2).
From the tens of thousands of wild shell fished just prior to the team’s arrival aboard the Paspaley vessel, three were discovered to have natural pearls still present within their sacs in the mantle, positioned in front of the gills and closest to the widest part of the adductor muscle (again, see figure 12). Upon inspection, we found that these shell had not been opened beyond the normal “natural relaxed” position. All three shells, and indeed all other wild shell aboard the vessel, were in the size range allowed for fishing wild shell for pearl culture (120–165 mm DVM; again, see figure 21). The three containing natural pearls ranged from 132.96 to 138.64 mm DVM and weighed (after cleaning) between 242.8 and 258.8 grams. The opening of the shell and the extraction of the pearls were witnessed by all members of the team. Both video and still images were recorded, and neither the shell nor the pearls have left the full
control of the team since that time.
The three natural pearls extracted (figure 24) weighed between 6.128 and 13.596 grains, with minimum to maximum dimensions of 5.93 and 8.20 mm. Their shapes were near round, button, and near oval. The control numbers for each of these three shell and pearls are 1WU, 2WU, and 3WU. None of these three shells had been operated on for pearl culture or any other purpose prior to the discovery of the pearls. A pearl weighing 35.04 grains was found in another wild shell, but in this instance the shell had previously been operated on and had been on the farm for more than a year (figures 25 and 26). As with the three previous discoveries, the pearl was found within the mantle, positioned in front of the gills and near the widest part of the adductor muscle. The shell was considerably larger than the three unoperated shells, with a DVM of 200 mm and a cleaned weight of 775.6 grams, nearly three times the weight of the largest wild unoperated shell. The pearl was almost 2.6 times the size of the largest specimen found in the wild unoperated shells. The control number for this pearl and shell is 1WO.
Four other pearls were discovered during this investigation.
The technicians aboard the vessels were aware of our interest and were on the lookout for anything unusual. In the first instance, one of the staff emerged from the operating room with a small dark pearl that had just been extracted from a hatchery shell that had yet to be operated upon. This pearl (4HU; figure 27) was rather small, measuring 3.10 × 2.43 mm and weighing only 0.74 grains. In the second occurrence a hatchery shell, also yet to be operated upon, was brought out with three pearls in the mantle. This time the pearls were located close to the heel of the shell rather than in front of the gills, as with the wild shell. The three pearls—one round, another round but with a slight drop shape, and the other a high button—weighed 6.784, 6.04, and 2.904 grains, respectively (figure 28). The control numbers for these pearls were 1HU, 2HU, and 3HU.
All microradiographic images from the examination of the pearls and shells were obtained with the Faxitron CS-100, a high-resolution real-time 2D Xray unit installed in GIA’s Bangkok laboratory. The samples were also examined using X-ray computed microtomography with a Procon X-rays CT-Mini model, also in the Bangkok laboratory.
The pearls and shell were examined using Gemolite microscopes at 10×–60× magnification. Photomicrographs were recorded digitally using a Nikon system SMZ1500 with a Nikon Digital Sight Capture System and at various magnifications up to 176×.
The chemical composition of the pearls and shell were determined with a Thermo X Series II laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP-MS) system equipped with an attached. New Wave Research UP-213 laser. UV-visible reflectance spectra for all samples were obtained with a Perkin”Elmer Lambda 950 UV-Vis-NIR spectrometer using a reflectance accessory bench fitted with an integrating sphere to capture data. Both Raman and PL data were recorded using a Renishaw inVia Raman microscope system incorporating a 512 nm
argon ion laser. All instruments are installed in GIA’s Bangkok laboratory.
OBSERVATIONS AND RESULTS
Selected microscopic images are shown in tables 3–7. As expected, the horny exterior of the shells hosted many foreign life forms taking the shapes of calcified undulating tubes (table 5F) coral exoskeletons (tables 3F, 4F, and 5E), or other unknown forms. We noted that the hinge of one shell also acted as the sarcophagus of a shrimp-like encrustation (table 6F), while a worm-like blister was apparent in shell 2WU (see table 4E). In each case, the shell had three major components: the non-nacreous edge, the nacreous inner core, and the hinge (tables 3A-3B, 4A-4B, 5A-5B and 6A-6B), all of which were characteristic in their appearance.
The non-nacreous edge under magnification revealed a clear prismatic growth in cross-section when viewed directly from above; the appearance differed slightly between reflected and transmitted light (tables 3D, 4D, 5D, and 6D). The nacreous central region, which was solid and had a naturally high luster, revealed the expected structure of overlapping platelets (tables 3C, 4C, 5C and 6C) when viewed at high magnification and in the ideal reflective lighting.
Magnification of each pearl, regardless of the source (wild or hatchery), revealed the expected overlapping platelet structures typical of nacreous pearls, both natural and cultured (tables 3I-3J, 4I–4J, 5I–5J, 6I–6J, 7B–7C, 7H–7I, 7J–7K, and 7P–7Q). In these instances, though, the structures observed in the pearls from hatchery shell (table 7) appeared somewhat coarser than those produced in wild shell. Microradiography and Micro-CT. Dubois (1901) suggested the use of X-rays (radiography) for detecting pearls in oysters and ably demonstrated the technique a decade later (Dubois, 1913). But it was not until the introduction of the round cultured pearl (Mikimoto, 1922) that the importance of X-rays as a gem identification tool was realized. Three X-ray techniques were applied to pearl identification. One in particular, microradiography, proved the most versatile (Alexander, 1941).
Since the advent of X-rays in pearl testing, there have been many technical advances, particularly in the areas of imaging and computerization. While film photography is still used as a backup, many gem laboratories today employ the more convenient highresolution 2-D real-time options, along with 3-D X-ray computed microtomography (micro-CT). Both real-time microradiographs and micro-CT images were recorded for pearls 1WU, 2WU, and 3WU (from wild unoperated shell). For the first sample, microradiographs recorded only the vague appearance of
an organic area toward the center of the pearl in one direction but a clearer image of this small centralized structure revealing micro “growth rings” was produced from another direction (table 3L). This sample was otherwise free of growth structures when microradiographs were taken in any direction. 3-D micro-CT scans revealed structures similar to those seen in the 2-D microradiographs. Zoomed-in areas of selected slices from the X, Y, and Z directions are shown in figure 29.
For pearl 2WU, the microradiographic detail was pronounced. A relatively large area of organic growth extended from the center of this 8.34 mm buttonshaped pearl to encompass about one third of the sample’s apparent volume. Within the dominant organic core, additional ringed growth structures could be observed toward the center of the pearl. Overall, the microradiographic structures revealed a great deal of organic material toward the center, while the outer portions appeared tightly crystalline with negligible organic material (table 4K–4L). 3-D micro-CT scans revealed structures similar to those seen in the 2-D microradiographs, but in slightly more detail. Zoomed areas of selected slices from the X, Y, and Z directions are seen in figure 30.
Pearl 3WU revealed little in terms of internal organic growth using 2-D microradiography (table 5K–5L). Under normal circumstances, therefore, one would regard this natural P. maxima pearl as “solid” Throughout. Yet 3-D micro-CT scans revealed two tiny points of organic accumulation not seen in the 2-D microradiographs. Figure 31 represents three slices, from the X, Y, and Z directions, that show these two dark spots quite clearly.
Pearl 1WO, which weighs 35.04 grains and measures 11.74 × 11.24 × 9.18 mm, was recovered from an older and larger wild shell than shells 1WU, 2WU, and 3WU described above. This shell had already been (gonad-) operated on for pearl cultivation and had been on the farm for about two years. The pearl was recovered from the mantle in a similar area to that of the other three.
2-D microradiography (table 6K–6L) revealed a slightly off-center area of patchy organic material in a P. maxima pearl that otherwise seems to be “solid” throughout. 3-D micro-CT scans revealed images similar to those obtained in 2-D, but in greater detail. While it is impossible to adequately reproduce the 3-D aspect of the micro-CT scans in the two-dimensional medium of this article, figure 32 presents three slices each from the X, Y, and Z directions. Viewing
these, one may surmise that the off-center area of patchy organic material is composed of many very small organic areas, both connected and unconnected with each other.
In table 7A, pearls 1HU, 2HU, 3HU, and 4HU present an interesting nomenclature dilemma: While they were found in mollusks that had not been operated on, these were hatchery-reared P. maxima. One school of thought suggests that as the host is “cultured” (i.e., hatchery-reared), anything that host produces should also be considered a product of culturing—i.e., a cultured pearl. As shown by the series of microradiographic images in table 7, however, nothing in their growth structures indicates a cultured origin. Indeed, all microradiographic indications point toward these pearls as being natural.
Not surprisingly, the microradiograph for pearl 4HU (which has a distinctly gray color) reveals the greatest amount of organic growth (table 7D–7E), and the pearl appears to have entirely natural growth structures. The microradiographs for pearls 1HU and 3HU (table 7L–7M and 7N–7O) reveal virtually nothing in terms of growth structures, which is expected for natural P. maxima pearls. Yet there were no indications that they were a product of culturing, either. Some of the microradiographs for pearl 2HU (table 7E–7G) did indicate a slight “shadowing.” As
with pearls 1HU and 3HU, however, the growth appears to be tight and crystalline. There is insufficient organic growth to appear on a microradiograph as diagnostic data. The same was also true for the micro-CT scans performed on each of these pearls.
Viewed under long-wave ultraviolet light, the pearls listed in table 2 showed a strong, fairly even chalky green fluorescence, and a much weaker chalky green under short-wave UV. The pearls were also examined using the DiamondView imaging system, which can produce a fluorescence image of the pearl in real time. The system uses a very short wavelength (below 230 nm) light source to excite fluorescence close to the surface of the pearl. These images have proved very useful in the detection of treatments, particularly coatings that are not visible under the microscope. The DiamondView images shown here (figure 33) will provide valuable reference data in future cases of treatment uncertainty. All three pearl types showed a distinctly blue fluorescence, sometimes slightly mottled, with no phosphorescence.
Raman and PL Spectra.
Raman spectroscopy is a technique in which photons of light from a laser interact with a material and produce scattered light of slightly different wavelengths. Every material produces a characteristic series of scattered light wavelengths, and measuring these can identify a material. The light of a particular wavelength from a laser beam (or other light source) is used to illuminate the gem. Because this laser light is aligned along the optical path of a microscope, the operator can focus it onto a gem to obtain a Raman spectrum (Kiefert et al., 2001). Light emitted by the sample is collected and analyzed by the spectrophotometer to produce a spectrum, which is compared to an extensive mineral database assembled by GIA over the past two decades.
Raman spectra recorded for the pearls listed in table 2 revealed two weak peaks located at 702 and 706 cm–1 (a doublet) and a strong peak at 1085 cm–1 (figure 34). These peaks are typical for aragonite, the crystalline material normally associated with pearls from P. maxima. No peaks associated with carotenoids or polyenes were recorded. No differences in the Raman spectra were noted between the three “types” of P. maxima pearls examined: from wild shell (unoperated), wild shell (operated), and hatchery-reared shell.
PL (photoluminescence) spectroscopy is a noncontact and nondestructive method used to probe the electronic structure of materials. In this process, a substance absorbs and re-radiates photons. It can be described as an excitation (in this study by a 514 nm argon ion laser) to a higher energy state, followed by a return to a lower energy state with the simultaneous emission of a photon (figure 35). The PL spectra can be collected and analyzed to provide information about the excited states, in this case by using the same system used to collect Raman spectra. No differences in the PL spectra were noted between P. maxima pearls from wild shell (operated or unoperated)
and hatchery-reared shell.
UV-Visible Spectroscopy.UV-Vis-NIR spectroscopy is a complementary technique to EDXRF for examining the trace-element composition of gems, particularly when detailed in absorption coefficient. UV-Vis-NIR spectroscopy may provide information about the portions of the visible spectrum that are absorbed by these trace elements to create the gem’s color. Given the opaque nature of pearls, such spectra must be recorded in a percentage reflectance. These spectra are important in defining some species and in some cases whether or not a treatment has been applied.
The white pearls in this group for which spectra were recorded (table 2) revealed curves that differed only in the reflectance at given wavelengths (figure 36). The only exception was 2WU, where there appears to be a slight difference in shape throughout the visible range (nominally 400–700 nm). The percentage reflectance throughout the visible region for each of the other samples decreases slightly toward the longer wavelengths. For sample 2HU, this translates to a percentage reflectance of 77.2 at 400 nm to 72.7 at 700 nm. For 1WO, this translates to a percentage reflectance of 84.65 at 400 nm and 78.41 at 700 nm. A reflectance trough at 278 nm is common to all the spectra for these pearls, as is a peak at 253 nm and a percentage reflectance drop to between 32 and 34 at 200 nm.
Chemical Composition. LA-ICP-MS provides qualitative and quantitative data of chemical elements. The laser sampling area (5 !m) can be focused on very small color and other surface zones. The ablation mark is less than the width of a human hair, visible only under magnification. The ablated particles are carried by helium gas to the plasma torch and mass spectrometer for analysis. The plasma unit atomizes and ionizes the atoms. The mass spectrometer measures the mass of each element for iden ti fi cation according to mass-tocharge ratio. LA-ICP-MS is a powerful method in the separation between saltwater and freshwater pearls and the detection of some treatments.
All of the pearls listed in table 2 were analyzed by LA-ICP-MS, and the results are presented in table 8. The pearls show great similarity in trace-element levels, with only 1WO trending toward the high end for Mn, Sr, Ba, La, Ce, and Pb. Many more examples of each type will need to be analyzed to determine if any significant trends exist.
The foregoing text and images clearly establish the ongoing recovery of natural pearls from P. maxima in Australian waters, a region with a significant pearling tradition stretching back to the 19th century and earlier (figure 37). The historical evidence is contained within official records as well as personal experiences related by respected authors of the time, such as Kornitzer (1937) and Kunz and Stevenson (1908). Many gemologists have written excellent papers on the separation of cultured from natural pearls using various techniques (see Recommended Reading list), but few have been wholly educational or all-encompassing in terms of the microradiographic structures one might expect from natural pearls. This may be because of the exceedingly wide variation of possibilities, the difficulty of gaining sufficiently high-resolution images, or the research time to devote to a project that produces a large volume of data.
Moreover, the journals would have to be willing to publish the extraordinary numbers of images necessary to convey the scope of the data. Web publishing is beginning to provide a greater volume of microradiographic structural images, which were and are beyond the scope of printed journals or books. An example of this is the document authored by N. Sturman (2009). Sturman (2009) shows through a series of microradiographs both obvious and subtle examples of internal structures recorded for non-bead (intentional or unintentional) cultured pearls. The paper also presents a few historical microradiographs for both natural and bead cultured pearls. Of the eight natural pearls collected during this project, samples 4HU (found in a hatchery unoperated shell), 1WU (taken from the mantle of a wild unoperated shell), and 2WU (from the mantle of a wild unoperated shell) may have sufficient internal growth structures to be identified as natural in a “blind” test.
Pearl 1WO (from the mantle of a wild operated shell) may not have a classic microradiographic structure for a natural or nonbead-cultured pearl, which might result in some debate concerning its nature given that the mollusk had been on a farm. Nevertheless, a blind test would conclude that the pearl was of natural origin, a result that would be consistent with the data collected. Returning to 3WU, the microradiographic structure recorded may easily misinterpreted as that of a nonbead-cultured pearl, and herein lies the first dilemma for those involved in both the pearling industry and pearl testing.
Over the past decade or so, the type of structure observed in pearl 3WU has been assumed to be an indicator of non-bead cultured growth. This assumption probably resulted from the structure’s resemblance to the “classic” nonbead-cultured pearl structure (see Sturman, 2009). This pearl challenges that assumption. The second dilemma concerns more the pearling industry. In industry discussions, it has often been suggested that anything produced by a mollusk on a pearl farm is cultured—and that a pearl produced by a hatchery-raised mollusk should also be considered cultured. Yet the very basis of a pearl culturing operation lies in the ability of technicians to create a
“sac” for the cultured pearl. It is not the host mollusk but the creation of this sac that defines the process. Pearls produced within a sac that is a product of human intervention are clearly cultured. But if a sac is a creation of nature, without human intervention then logic dictates that anything it produces is “of nature.” Even if one opposes this logic, the fact remains that pearls 1HU, 2HU, 3HU, and 4HU, the products of pearl sacs formed by nature within hatchery-
reared shell, are virtually indistinguishable from natural pearls and could not be identified as cultured.
This examination of a small number of definitive samples has therefore produced what may appear to be unexpected results that may add further to the challenges faced with pearl identification. Clearly, many more samples from each of the types discussed will need to be collected and examined before a clearer picture emerges. In the meantime, the authors will conduct ongoing expeditions and research. In late November 2012, some of the authors were able to extract another 30 natural pearls from Australian Pinctada maxima, and the technical data from these will be the subject of another report.
Articles source: Kenneth Scarratt, Peter Bracher, Michael Bracher, Ali Attawi, Ali Safar, Sudarat Saeseaw, Artitaya Homkrajae, and Nicholas Sturman – GEMS & GEMOLOGY, WINTER 2012 For Questions and answer you can contact & chat with us on: