Vaterite influences lustre and quality of freshwater cultured pearls

WEHRMEISTER, URSULA; JACOB, DORRIT E.; SOLDATI, ANALIA L.; HÄGER, TOBIAS; HOFMEISTER, WOLFGANG

Bangkok and Kanchanaburi, Thailand

Conferencia; The 2nd International Gem & Jewelry Conference (GIT 2009); 2009

Vaterite, a metastable CaCO3 polymorph, has recently been found in the upper layers and at the surface of freshwater cultured pearls. Vaterite has a different crystal structure to aragonite and lacks the pearly lustre, when situated at the surface of the pearl. Here, we present a systematic account of the occurrence of vaterite in good quality freshwater cultured pearls and show that vaterite is not restricted to lacklustre pearls and, thus, represents an important factor in influencing the quality and value of pearls. Pearls and shells of molluscs are products of biomineralization – formed by a living organism. The epithelian cells of the mollusc´s mantle tissue are responsible for the formation of the crystals, both of shells and pearls. The framework in which the crystals are formed is provided by an organic matrix. The organic macromolecules that are partly included in the crystal structure during growth have been found to play a key function in controlling the crystal growth process. In general, beaded and non beaded freshwater pearls can be cultured by implanting a piece of ephitelian tissue either in the gonad or in the mantle tissue of the mussel. This piece of tissue has the key role to stimulate the formation of the pearl sac, in which the pearl is subsequently built.