Pearls are the most valuable of all organic gemstones, and have been prized for 6000 years. Pearls chemical makeup varies by region and environmental factors, but the basic formula is CaCO3. Pearls form through irritants entering specific species of mollusks (oysters, clams, muscles, etc), while the irritant can be a grain of sand or a parasite, the process is the same for pearls. The irritant is coated in CaCO3(mother of pearl) to make it less irritating to the mollusk. Pearls can range in many colours and shapes, and value depends on their shape and colour.