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
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