The length of time needed to form a pearl varies with the species. Clams, mussels and oysters are bivalves within the mollusk family that create pearls by surrounding an irritant in the organism's outer tissue with nacre, using layers to make a pearl.
Once the irritant comes to rest in the bivalve's tissue, the bivalve begins secreting nacre, a combination of conchiolin and calcium carbonate, in thin layers to build a barrier between the irritant and the bivalve's tissue. In most species, the rate of nacre deposit is about 0.6 millimeters per year, but that varies by species. The aragonite that forms within the nacre is the source of the luster and iridescence that gives a pearl its beautiful glow.