Pearl is a biogenic precious gemstone produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers, as a defense mechanism against a potentially threatening irritant such as a parasite inside its shell, injuring the mantle tissue. The mollusk creates a pearl sac to seal off the irritation. The process through which the pearls are formed inside the body of the mollusk is very delicate, and a little disturbance at any stage of development of a pearl can influence its shape and luster. The finest pearl is that which has no foreign matter in its core.