Formed from the soft tissue of a shelled mollusk, pearls are made up of layers of calcium carbonate. Pearls should ideally be round and smooth, but many other shapes exist. The finest of pearls have been considered gems and are looked on as things of great value, beauty and rarity. The Pearl Source is an example of a company that distributes pearls in the form of necklaces, earrings, pendants, bracelets and rings. The necklaces, for example, come in several different milliliter thicknesses. Earring studs may come in different colors and shades to match different decor or outfits. This company has