How cultured pearls acquire their colour

Round nucleated pearls are produced through a surgical operation, where a round nucleus and a mantle tissue 'saibo' from donor oyster are inserted into the gonad of the host oyster. The epithelial cells in the mantle tissue proliferate around the nucleus, and thus, the pearl sac is formed. Pearl sac secrets nacre and forms a pearl. The quality and economic value of pearls are assessed by pearl features such as colour, brightness, lustre and shape. Among all these features, colour has been reported as an important economic indicator and has been widely studied by researchers. Generally, pearl colour is affected by the donor oyster which is determined genetically and biological pigments (melanin and carotenoid). Organic matrices, metal ions and other factors have also been reported to influence the colour of a cultured pearl. Recently, multi-omics methods have been used to study the colour formation of pearl, and some key genes and signal pathways related to the colour formation of pearls have been identified. Nevertheless, the specific mechanism of pearl formation needs further research. The review combines both fresh and sea water pearls focusing onHyriopsis cumingiiand pearl oysters to provide a general overview and understanding for pearl colour formation.