What do foreign particles turn into inside a shellfish shell?

Sometimes foreign particles, usually a grain of sand or a parasite, get in and get stuck between the mantle and shell of the mollusk. The unpleasant sensations arising at the same time force him to isolate the grain of sand with a crust of mother-of-pearl, a limestone substance made of calcium carbonate. Several layers of mother-of-pearl form a lustrous bead – a pearl, the color of which can be from white to black. The main pearl producers are marine and freshwater bivalve molluscs.

In some countries, such as Japan, pearl production is organized by artificially introducing foreign bodies into bivalve molluscs.