Pearls are formed when an oyster coats any hard particle entering its shell with layers of lustrous nacre, a form of calcium carbonate. The Japanese cold-water akoya oyster ( P. fucata ), smaller than the tropical P. maxima , produces pearls up to 9 mm in diameter. These are usually cultured using round nuclei, though natural, non-nucleated, irregularly-shaped pearls (called keshi or ‘poppy-seed' pearls) occur in all oyster species. Australian cultured pearls begin at 10 mm in diameter and occasionally grow to larger than 20 mm. South Sea pearls from WA dominate the high quality large pearl market. Oysters are initially seeded with nuclei made from Mississippi mussel shell which has sufficient thickness to produce large round nuclei. A piece of mantle (the tissue which produces nacre) from another oyster is inserted with the bead into a surgically-created pocket in the animal's gonad. In an oyster's last year of production, a half-round nucleus is often used to produce a blister pearl (called a ‘mabe') on the shell wall. Round pearls are normally used for necklace strands and while keshi can also be strung, they and mabe are generally set in jewellery. | 