The pearl is unique among all other gemstones in the world. It is the only gem that is grown inside a living organism

The lustrous outer surface, or nacre, of natural and cultured pearls is made up of the same material. Its chemical composition is about 90% calcium carbonate; the rest is water and organic materials. Pearl nacre is composed of microscopic crystals. The crystals are aligned perfectly so that light passing along the axis of one is reflected and refracted by the others to produce a rainbow of light and color. The iridescence that we commonly associate with pearls is produced by this arrangment of layers.