Mother Of Pearl
Mother Of Pearl Forms The Inside Of Shells
Famed for its shimmering beauty the world over, 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- also known as luster- is an optical phenomenon wherein wavelengths of light are scattered and refracted back toward the viewer in a dazzling display.
Until it was replaced by plastic in the mid-20th century, mother-of-pearl was also used to produce shiny buttons for clothing. This was the case in Broome, Australia, a well-known South Sea pearl producing area. Before South Sea pearls became the area staple, this small town thrived on the Pinctada maxima mother-of-pearl business.
Beauty And Purpose In The Sea
Mother of pearl has two functions: it provides protection from predators, and provides shelter from the elements. The shielding of the mollusk from predators and parasites is a process called encystation. Nacre secreted by the mollusk traps and entombs invading parasites and foreign deitrus, smoothing over the irritant and sometimes creating the rarest of treasures: a pearl.
The Growth Of A Shell
Nacre provides shelter for the soft-bodied mollusk; the strength and resiliency of 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 of nacre that will thicken and widen outward continually, to accommodate the animal throughout its lifespan.
The Shell Structure
The mollusk’s shell contains three layers: the periostacum (the conchiolin layer), the ostracum (the prismatic layer), and the hypostracum (the final mother of pearl layer).
The Protective Outer Shell
The periostracum is the outer layer of the shell; this is the first layer to form around the mollusk during its immature, glochidaeal stage, and is composed entirely of organic conchiolin, the dark brownish, black, green or dark blue-colored organic substance made up of keratin proteins which are also be found in the human epidermis.
The Prismatic Layer
The ostracum, or prismatic layer, is the central layer and is composed of tiny hexagonal calcite crystals measuring one micron in size. In freshwater mollusks the crystals are mainly aragonite, due to variations of trace elements found in the water. Contrary to its name, the prismatic layer is neither iridescent nor beautiful; it provides a level of stability to the hypostracum, and is brown to 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
The hypostracum, or mother of pearl layer, is the final layer that displays color and iridescence. Mother of pearl is almost 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 similar to topographical maps detailing ridges and valleys when viewed under 40x magnification.
Growth of the mother of pearl layers is very 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 back 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.