The scallop is a well-known bivalve mollusk prized for its adductor muscle and recognized by its distinctive, fluted shell. Like all mollusks, scallops can respond to an internal irritant by secreting mineral compounds. While this defense mechanism leads many to wonder if they produce valuable pearls like oysters, the resulting formation is not a traditional, commercially viable pearl.
The Direct Answer: Pearl Production in Scallops
Any mollusk can secrete material to encapsulate a foreign body, but the resulting structure in a scallop is exceptionally rare. These formations are known as calcareous concretions, not true pearls, because they lack the iridescent sheen. They are an accidental byproduct of the wild-harvesting industry, with no commercial fishery existing to seek them out. Estimates suggest that only about one in 10,000 scallops may contain one, and few are of gem quality.
Scallop concretions are non-nacreous and mostly opaque, ranging in size from a tiny seed to over 40 carats. While often white or brownish, rare specimens have been found in shades of purple, orange, or deep maroon. These concretions may exhibit aventurescence, a visual effect where tiny internal planes reflect light to create a three-dimensional shimmer.
Anatomy and the Nacre Barrier
The reason scallops rarely produce gem-quality pearls is rooted in their biological architecture. Pearl formation is initiated by the mantle tissue, the organ responsible for secreting shell material. In pearl oysters and certain mussels, the mantle secretes nacre, which lines the inner shell. This allows them to coat an irritant with the lustrous material that forms a true pearl.
Scallops belong to the majority of bivalve species whose shells do not possess a nacreous inner layer. Instead of specialized nacre-producing cells, the scallop mantle secretes different forms of calcium carbonate. This structural limitation means the mollusk cannot deposit the layered, iridescent material around an irritant.
Defining True Pearls and Calcareous Concretions
A defining feature of a “true pearl” is its composition of nacre, a composite material secreted by the mollusk. Nacre is chemically composed of hexagonal platelets of aragonite, a crystalline form of calcium carbonate. These organized platelets are held together by an organic protein binder called conchiolin, creating a layered structure. This architecture interacts with light to produce the deep luster and iridescence, or orient, seen in valuable pearls.
The formations found in scallops are properly termed calcareous concretions because they lack this nacreous structure. These concretions are also calcium carbonate, but they are composed of calcite, a different crystalline form. They lack the fine, parallel layers of aragonite that give a true pearl its shimmering surface. Without this ordered microstructure, the scallop’s concretion has a porcelain-like or chalky appearance, differentiating it from the highly prized pearls of the Pinctada genus.