Pearl Composition and Structure

Mollusks filter microscopic food from the water they live in. The water is also the source of the chemical elements they use to make their shells, bodies, and organs. One of the organs is the mantle. It’s similar to skin and forms the lining of the mollusk’s shell. Cells on the shell side of the mantle secrete complex liquids that harden to build the shell. One of those hardened liquids is called mother-of-pearl. When mother-of-pearl composes all or part of a pearl, it’s called nacre. 

Nacre is made up mostly of conchiolin and layers of calcium carbonate platelets. Conchiolin and its related substances are proteins. They control platelet production and also form a matrix that holds the platelets together. Depending on structure, the calcium carbonate platelets might be aragonite or calcite, or a mixture of both.

Aragonite and calcite are both composed of the same elements: calcium, carbon, and oxygen. But they have different crystal structures. This makes a difference in the way they interact with light. This interaction contributes to the quality of a pearl’s appearance in reflected light, known as luster. Aragonite crystals and light interact to create complex luster, while calcite crystals and light create a simpler shine that’s similar to porcelain.

Another effect of nacre’s extremely thin layers is that white light sometimes travels through it at different speeds and in different directions. This causes light to break up into its component spectral colors, creating an iridescent rainbow effect. This effect is called orient when it’s seen in a pearl or in mother-of-pearl. The presence and degree of orient depends on the pearl type and its shape, the characteristics of its nacre, and the viewing angle and surroundings.

Valerie Power/GIA
Chinese freshwater cultured pearls are composed almost entirely of nacre. They contain mostly aragonite, with some calcite and organic material.