Pearl formation and pearl characteristics
Based on its composition, pearls can be categorised as nacreous and non-nacreous (Kennedy, 1998). Nacreous pearls (mostly aragonite) are produced by species with nacreous lining to the inner surfaces of their shell(s) (MOP) but non-nacreous pearls can be produced either from non-nacreous shells or from nacreous shells that may secrete pearls with less aragonite platelets. Commercially cultured pearls, however, are mostly nacreous. The following description of pearl and nacre formation is based on studies with nacreous shells or pearls.
Cultured pearl formation begins with the development of a pearl-sac that is formed from proliferation of saibo tissue (Scoones, 1996). This is the tissue responsible for nacre secretion. Along with the development of the pearl-sac, mineral deposition occurs and continues after the mantle heals or forms a sac. The process of pearl-sac formation and mineral deposition may take up to six months after the implantation in P. margaritifera (Haws, 2002) and the complete healing of the pearl-sac in P. martensi can be within two weeks only (Strack, 2006).
Scoones (1996) studied pearl-sac formation and mineral/nacre deposition in P. maxima in detail. He reported that development of the pearl-sac took approximately 23 days from the implantation and that the first secretions from the pearl-sac onto the nucleus were evident about 30 days after the implantation. Mineral deposition within the pearl-sac begins with the secretion of periostracum and is followed by ostracum for non-nacreous pearls.
In nacreous pearl formation however, the layers of periostracum and ostracum are covered with a hypostracum (nacre) layer. The pearl formation mechanism follows the layering structure of the shell but in reversed order (Strack, 2006). In the shell, the periostracum forms the outer surface while it is the innermost layer at the interface between the nucleus and the pearl layers, in a cultured pearl.
The periostracum is a thin layer that contains mainly conchiolin. The other layers: ostracum or prismatic layer and the hypostracum or nacreous layers are two polymorphous layers of calcium carbonate. These two calcareous layers are composed of calcite (in the prismatic layer) and aragonite (in the nacreous layer). The building structure of the prismatic layer is columnar while the nacreous layer is composed of layers in a brick-mortar arrangement where the bricks are aragonite platelets and the mortar is composed with organic matrix (Addadi & Weiner, 1997; Barthelat & Espinosa, 2007; Checa & Rodriguez-Navarro, 2005; Fougerouse et al., 2008; Gre´goire, 1957; Katti & Katti, 2006; Rousseau et al., 2004) (Fig. 1.6).
A study with Pinctada maxima, reported that the uniformity of the nacre structure may contribute to the saturation of pearl colour; the more regular the structure the more saturated the colour will be (Snow et al., 2004). However, most of the studies on pearl layers have mainly focused on shell nacre (MOP) as a parallel comparison to the pearl. There is a difference between the structure of nacre in pearls and that in shells: nacre is concentric in pearls while it is layered in MOP (Strack, 2006). Recent detailed studies on pearl structure are few.
Fig. 1.6. Schematic drawings of (a) a three dimensional view of brick-mortar arrangement of aragonite platelets and conchiolin as a coating matrix, and (b) a cross-section view of the brick mortar arrangement of nacre in a pearl (Taylor & Strack, 2008).
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