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  • Nacre

    Pearl Nacre Defined

    Nacre, also known as mother-of-pearl, is a crystalline substance that creates the iridescent visual effect attributed to pearls.
    Different Species of Pearl producing mollusks


    Nacre is an organic substance secreted by mollusks to produce their shells. It may be used to protect the animal against an intruder (the so called "irritant") or an implanted nucleus in pearl farming.

    It is a strong and resilient material that is lightweight, reflective and translucent, allowing light to pass through its surface as well as to bounce off from it, this creating both the subtle glow on the pearl's surface as well as its iridescent look.
    Click image for larger version

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    Nacre's Composition

    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.
    Click image for larger version  Name:	PearlInfo-Nacre-CrossSection-WEB.jpg Views:	2 Size:	11.0 KB ID:	449868
    Image courtesy of PearlParadise.com


    Nacre is Composed of Aragonite Platelets

    Pearls and Mother-of-Pearl are made from Calcium Carbonate, which exists naturally in two different forms: Calcite and Aragonite. The shells of most mollusks (those without mother-of-pearl shell) are made from Calcite, and fewer will produce and display Aragonite, which forms hexagonal shaped crystals.

    Aragonite in its crystal form can be found all over the world, however due to its rating of 3 to 4 under Moh's Scale, it is considered a soft crystal. But, when combined with the organic biopolymers and proteins that the mollusk naturally secretes, the substance becomes nacre, which is remarkably strong and rivals silicon in hardness (6 to 7 in Moh's scale).

    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.
    Nacre Aragonite Platelets Crystals. Photo by SSEF



    A Mollusk Secretes Nacre to Produce 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.
    Pteria Sterna Spirals 100x by Lore Kieffert SSEF


    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 are comparable to a wavelength of white light, which makes the transmission and refraction of light rays possible throughout the crystalline layers. Upon encountering the surface of the pearl, a significant percentage of white light can penetrate the top layer and work its way through the nacre strata.

    Each platelet that the light encounters act 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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    Last edited by CortezPearls; 03-23-2021, 08:00 PM. Reason: Improved text & added images
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