Scallops are widely recognized bivalve mollusks, primarily known as a delicacy prized for their adductor muscle, not as a source of jewelry. However, like many shelled creatures in the phylum Mollusca, scallops possess the biological mechanism to form a concretion around an invading irritant. The resulting object is a pearl, though its appearance and composition differ significantly from the iridescent gems typically harvested from pearl oysters.
The Unique Shell Structure of Scallops
Scallops belong to the family Pectinidae, which are bivalve mollusks characterized by their distinctive, fan-shaped, ribbed shells. The shells of all mollusks are formed by the mantle tissue, which secretes layers of calcium carbonate. In commercially valued pearl oysters, the innermost layer of the shell is lined with nacre.
Nacre is a composite material consisting of microscopic hexagonal platelets of aragonite, a form of calcium carbonate, cemented together by an organic protein called conchiolin. This layered structure is responsible for the iridescent sheen, or orient, seen in classic pearls. Scallops, in contrast to pearl oysters, typically lack this extensive nacreous inner layer.
The lack of a nacreous lining means the scallop’s mantle tissue secretes different forms of calcium carbonate. Instead of the overlapping aragonite platelets of nacre, the scallop’s shell is often composed of porcellaneous, or chalky, calcium carbonate or other crystalline structures like calcite. This difference in shell composition dictates the unique nature of the pearls they produce.
The Mechanism of Pearl Formation
Pearl formation is a defense mechanism triggered when an unwanted foreign object enters the mollusk’s shell. This object, such as a grain of sand or a parasite, becomes lodged between the shell and the mantle tissue. Since the animal cannot expel the intruder, it isolates the irritant.
To neutralize the threat, the mollusk’s mantle tissue envelops the irritant, forming a protective structure called a pearl sac. The cells of the pearl sac then continuously secrete layer upon layer of the same material that forms the animal’s shell. This layered secretion builds up around the foreign particle, gradually forming a pearl.
The final composition of the pearl depends entirely on the material secreted by the mantle. In oysters, the pearl sac secretes nacre, resulting in a lustrous gem. In scallops, the mantle tissue secretes other crystalline forms of calcium carbonate, such as aragonite or calcite, without the organized, plate-like structure that creates iridescence.
Scallop Pearl Appearance and Rarity
Scallops produce concretions that are correctly called pearls, but they are classified as non-nacreous. Since their shells lack a nacreous layer, their pearls are composed of dense, porcellaneous calcium carbonate. These non-nacreous pearls lack the characteristic iridescent sheen, or orient, of traditional pearls, appearing more opaque and dull.
The appearance of scallop pearls is distinct, often exhibiting a matte, porcelain-like finish. They can range in color from white and tan to unique shades of orange, pink, and deep purple. Some specimens, particularly those from the Pacific Lion’s Paw scallop (Nodipecten subnodosus), display a unique optical effect known as aventurescence. This effect gives the surface a three-dimensional, shimmering quality due to a fibrous structure beneath the surface.
Scallop pearls are extremely rare, making them an unusual byproduct rather than a commercial target. Estimates suggest that only about one in every 10,000 scallops may produce a pearl, and even fewer produce one that is gem-quality. They are typically discovered accidentally by fishermen harvesting the scallops for food, which contributes to their high value among collectors and gemologists.