Nacre, also known as mother-of-pearl, is a crystalline substance that creates the iridescent visual effect attributed to pearls. Nacre is an organic substance secreted by mollusks over an intruding irritant or implanted nucleus. It is a strong and resilient material that is lightweight and transparent, allowing light to pass through its surface, creating a subtle glow on the pearl’s surface.
The Composition Of Nacre
Nacre is composed primarily of crystallized calcium carbonate (CaCo3) and conchiolin. Conchiolin is a dark-colored substance secreted by the mollusk during the initial phases of pearl formation. Conchiolin is an organic protein that acts as a form of glue or adhesive. Generally it is the first layer deposited by the pearl sac; conchiolin surrounds the bead nucleus or irritant and functions as a base coat that will cause the pursuant nacre layers to bind together. The brownish material does not always confine itself to the initial coating, and may be observed throughout the nacreous stratum in cultured pearls that have been cross-sectioned.
Image courtesy of PearlParadise.com
Nacre Is Composed Of Aragonite Platelets
Crystallized calcium carbonate contains millions of aragonite platelets, elastic biopolymers such as lustrin and chitin, and silk-like proteins. Aragonite platelets are so miniscule they can only be viewed using a minimum power 2000x electron microscope; hexagonal and polygonal in shape, the average size of a single platelet ranges between 0.35-0.5 microns thick and 3.0-6.0 microns across. Aragonite in its crystal form can be found all over the world, however due to its low 3-4 Moh’s Scale rating, it is considered a soft gem; when combined with the organic biopolymers and proteins that the mollusk naturally secretes the substance becomes remarkably strong and rivals silicon in durability.
A Mollusk Secretes Nacre To Form A Pearl
The mollusk continues to secrete concentric layers of nacre around the nucleus at irregular intervals; microns-thin sheets of crystalline material are laid down in starts and stops and do not fit perfectly together, resulting in a delicate lacework of ridges and swirls similar to a topographical map detailing mountainous regions and valleys. These patterns are often referred to as the pearl’s own fingerprint; each layer is singularly unique in composition and has no equal.
Nacre Is The Beauty Of A Pearl
The exact shape and size of the aragonite platelets are ultimately what influences a pearl’s two most important quality attributes: luster and orient. The platelets’ own thinness and transparency is comparable to a wavelength of white light, which makes the transmission and refraction of light rays possible throughout the crystalline layers. Upon coming into contact with the surface of the pearl, a significant percentage of white light is able to penetrate the top layer and work its way through the nacre strata. Each platelet that the light comes into contact with acts as a tiny prism, breaking up the beam and refracting back a subtle rainbow of color that is visible on the surface of the pearl.
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