Initial formation stage and succedent biomineralization of pearls

• A new initial formation mineral phase was discovered in pearl.
• Nanograins and nanostripes inside aragonite platelets were found in pearl.
• Two possible initial formation stages were proposed for pearl.
• A possible biomineralization mechanism of nacre in pearl was proposed.

The initial formation stage and succedent biomineralization of pearls were studied using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and atomic force microscopy. A new initial formation phase with needle-like structure which is found to be nanocrystallites of aragonite was discovered. As a result, two possible formation modes are proposed to describe the initial formation stage of pearls. As for the succedent mineralization of “brick and mortar” structure, nanostripes were first discovered inside the “brick” (aragonite platelet), compared with the foregoing finding of nanograins. The various nanostructures of aragonite platelet allow us to reconsider the role of the inter- and intracrystalline organic material surrounding CaCO3, and a possible biomineralization mechanism was proposed.