**5. Biomineralisation of pearls**

Biomineralisation is a common occurrence in a variety of species [66]. A pearl is a well-known organo-mineral composite product of biomineralisation that is

#### *Recent Trends in Freshwater Pearl Farming in India DOI: http://dx.doi.org/10.5772/intechopen.99281*

composed of more than 95% calcium carbonate (CaCO3) and less than 5% organic molecules [6, 67, 68]. The process by virtue of which the pearl develops in mussel/ oyster is otherwise termed as biomineralisation and the outer epithelium of the mantle plays an important part in pearl biomineralisation. It is formed when an external stimulus like a foreign body or a parasite is fortuitously trapped in the bivalve followed by biomineralisation around the non-native particle via the deposition of pearl nacre in micro-layers. Nevertheless, this phenomenon is restricted to those bivalve species whose interior surface of the shell carries the nacreous layer. The peculiarity of pearl formation being restricted to certain species of bivalves can also be supported by the fact that a prominent resemblance in the CaCO3 and matrix protein component lies between the pearl and nacreous layer of the organism's shell.

Biomineralisation is attributed to a complex physiological process wherein various matrix proteins and calcium metabolism regulatory proteins secreted from the mantle epithelium act together to form the lustrous product that is preceded by the formation of the pearl sac around the foreign particle by the proliferation of the outer epithelium of the mantle [6, 7, 69–71]. The matrix proteins contribute to the pearl formation process in forming a biomineral framework and simultaneously regulating the nucleation as well as the growth of calcium carbonate crystals [6, 7]. Microstructure analysis of pearls reveals that the crystallising layers of the pearl include three polymorphs of CaCO3 that include aragonite, calcite and vaterite, which also govern the quality of pearls [6]. The mantle shell of molluscan species consists of three layers, the outer periostracum, middle prismatic and the inner nacreous layer [7, 72, 73]. The periostracum, a layer of strongly cross-linked proteins that covers the shell's external surface, is produced by the mantle [74]. Secretions from the edge region of the mantle (mantle edge) form the prismatic layer while the inner part of the mantle (mantle core) makes up the nacreous layer or "mother-of-pearl", which is the most widely studied structural motif. Studies on the crystal structures of molluscan shells reveal that the prismatic and nacreous layer consists of calcite and aragonite, respectively [67, 73] and this polymorph formation is regimented by the differences in the mantle epithelium's secreted protein [7, 67]. The third polymorph, vaterite is an unstable and rarely occurring form of CaCO3 that has been exclusively reported in freshwater pearl mussel species [6, 75, 76].
