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  • Mother-of-Pearl (MOP)

    Mother-of-Pearl Defined

    Mother-of-Pearl (MOP for short) or Nacre (as it is also known) is the iridescent coating on the inside of some mollusk species which is composed of aragonite and calcite, both being calcium carbonate polymorphs. MOP also contains water and binding proteins secreted by the mollusks, the most important being conchiolin.

    Mother-of-pearl is also the primary material used in pearl nucleus manufacturing. MOP beads are still the most common nucleus (core) used to produce saltwater cultured pearls.


    Mother-of-Pearl forms the Inside of Shells

    Famed for its shimmering beauty the world over, the iridescent shell known as mother-of-pearl is the organic/inorganic material lining the inside of a mollusk's shell.

    The play of color that floats over the surface of the interior lining of the shell is also known as iridescence, and this is an optical phenomenon wherein wavelengths of light are scattered and refracted back toward the viewer in a dazzling visual rainbow-like display.
    The beautiful iridescent colors of MOP shell (Pteria penguin)



    Commercial uses for Mother-of-Pearl

    Until it was replaced by plastic in the mid-20th century, mother-of-pearl was used to produce shiny buttons for clothing, fine dining utensils, knives and pistol handles, jewelry, cigarette and snuff boxes, mirror frames, jewelry, cameos, cufflinks, elegant pens and many, many other items.
    A checkerboard cigarette box made with white & black MOP shells.



    In the case of mother-of-pearl buttons, there are two towns that had thriving MOP button operations, one of these was Camden, Tennesse, in the United States, where pearly mussels were fished out to near extinction, but later had a revival with the establishment of a freshwater pearl farm owned by the Latendresse family. An interesting article can be found here.

    This was also the case for Broome, Australia, a well-known South Sea pearl producing area. Before South Sea pearls cultivation started, this small town thrived on the Pinctada maxima mother-of-pearl shell business.

    Mother-of-Pearl is used to Help Cultivate Pearls

    Mother-of-pearl is now used extensively as the "nucleus" or "seed" in pearl cultivation. The shell of a mussel is first cut into thick strips, then into cubes and then run through a process that rounds the pieces into round beads. Finally, the beads are polished and sorted by size and qualities, then sent all over the world for pearl farmers to use in their cultured pearl operations.
    Commercial MOP Nuclei



    These beads are implanted into the oysters' gonads alongside a small piece of graft tissue (known as "Saibo") which -if it survives and grafts into the surrounding tissues- will form a "pearl sac" which will then secrete nacre upon the mother-of-pearl beads to finally help create a cultured pearl. Read about Nacre for a more complete description of this process.


    Beauty and Purpose in One

    Mother of pearl serves two functions for mollusks: the most vital being that it provides them with a protective shell, which serves them against predators and provides shelter from the elements.

    The second one being for a process called encystation. This process is the secretion of nacre to "trap and entomb" invading parasites, and in the process sometimes creating the rarest of treasures: a natural pearl.

    But why do mollusks secrete MOP over the more conventional calcite shell? Evolutionary scientists argued that this variety of shell evolved in a moment in our planet's history when disolved calcium carbonate (in water, of course) was in high demand and short supply. Before this event, many species of mollusks were able of attaining enormous size!
    Kevin McLaurin next to a large fossilized clam shell



    Some mollusks decided (in a figurative manner) to find an alternative material and it was by means of Aragonite. Their shells would be much lighter and thinner, yet stronger at the same thickness ratio than those of mollusks using calcite shells. This was a great innovation, especially for those mollusks that protected their soft bodies with shell but also wanted to "swim" in those primitive, predator infested, water bodies, such as ammonites.

    To give you an idea of the superior resistance of MOP or nacre, consider this: In 2014, researchers used lasers to create an analogue of nacre by engraving networks of wavy 3D "micro-cracks" in glass. When the slides were subjected to an impact, the micro-cracks absorbed and dispersed the energy, keeping the glass from shattering. Altogether, the treated glass was 200 times tougher than untreated glass.

    The Growth of a Shell

    Nacre provides shelter for the soft-bodied mollusk; the strength and resiliency of their shell enables the animal to survive the unceasing environmental and tensile pressures endured in its habitat.

    The shell structure begins with an oval-like, rounded shape called the umbo. The umbo is gradually surrounded by concentric growth rings that will thicken and widen outward continually, to accommodate the animal throughout its lifespan. This is like the way a tree grows, with concentric rings growing outwards.


    The Shell's Structure

    The mollusk's shell contains three layers: the periostracum (the protein-based conchiolin outer layer of the shell), the ostracum (the middle prismatic layer), and the hypostracum (the innermost and actual mother-of-pearl layer).
    Exterior shells of different Pinctada species

    From left to right: Panamic black lip (Pinctada mazatlanica), Fijii black lip (P. margaritifera), Tahitian black lip (P. margaritifera cummingi) and Silver lip (P. maxima). You can see their exterior coloration and even their "growth rings".

    The Protective Outer Shell - Periostracum

    The periostracum is the outer layer of the shell; this is the first layer to form around the mollusk during its immature, larval stage, and is composed entirely of organic conchiolin and a brownish, black or green colored organic substance made up of melanin and quinone-like proteins, which are also be found in the human epidermis and cuticles.

    The Prismatic Layer - Ostracum

    The ostracum (Greek for "shell") or prismatic layer, is the central layer and is composed of tiny hexagonal calcite crystals measuring one micron in size. Contrary to its name, the prismatic layer is neither iridescent nor beautiful; it provides a level of resistance and stability to the shell and is brownish-yellow in color with a porcelain finish. The crystals are arranged parallel to each other and are held together by a thin film of organic material.

    The Nacreous Layer - Hypostracum

    The hypostracum, or mother-of-pearl layer, is the inner shell layer that displays color and iridescence. Mother-of-pearl is entirely made up of calcium carbonate (CaCo3), which contains millions of crystal aragonite platelets. These crystals measure 0.3-0.5 microns in diameter and are stacked on top of each other in a brick-like structure. Interspersed lie extremely thin sheets of conchiolin that "glue" the platelets together. Each layer grows intermittently; some layers show straight and even growth, while others are rounded, however none of them run directly parallel to the surface of the shell. The result is like topographical maps detailing ridges and valleys when viewed under 40x magnification.

    Growth of the mother of pearl layers is terribly slow. In P. margaritifera, the average rate is approximately 13 aragonite platelets per day; other species secrete nacre at faster or slower speeds depending upon environmental conditions.

    The Crystalline Effect

    Aragonite platelets are transparent and mimic wavelengths of visible light, enabling light rays to be absorbed, scattered, and reflected at the viewer in various hues resulting in the phenomenon of iridescence.

    The word iridescence comes from the Greek word iris and translates as "Rainbow". Grecian legend portrays the ancient goddess Iris personified as a shining arc of color announcing divine proclamations from Olympus to mortals who revered her as a spiritual advisor.

    While we now know the science behind the effect of iridescence, it does not make the phenomenon any less beautiful or inspiring to the imagination of people around the world.


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