Shell and Mother of Pearl
Shell used in jewelry come from mollusks. Mollusks (or molluscs) are the boneless animals familiar to beachcombers as “seashells” include animals like clams, oysters, mussels, snails, tusk shells, scaphopods, chitons, and cephalopods, like the nautilus and spirula.G11 The shell of the mollusk … the part used by so many Native American artists … is actually a calcareous exoskeleton that encloses, supports and protects the animal’s soft parts.
Most of the material sold on the market as “shell” comes from marine bivalves of one type or another. People know bivalves as the common seashells that wash up on ocean beaches or on banks of rivers and lakes. The sea obviously the primary source of shell however, freshwater lakes and rivers also produce important quantities of this organic material. Sometimes they’re so abundant that they make up a large portion of the beach substrate, as in Shell Beach, of Western Australia, where Cockle shells of a single species, Fragum erugatum make up beach.
The composition of shell makes it resistant and useful for jeweling. It’s made up of several layers. The hard, calcareous layer … the part that one usually finds on the beach once the shell has washed up on the shore … consists of a secretion of calcium carbonate (CaCO3). The bivalves deposit enough calcium carbonate in this layer to protect its soft innards, and it can be quite hard and thick in some species.G12
Just inside this hard, protective shell layer, one finds a smoother, nacreous layer (nacre rhymes with “baker”), known commonly as “Mother of Pearl.” It occurs mostly in the ancient families of mollusks, like the Top Snails and Pearl Oysters. Nacre’s the silvery, iridescent, smooth inner surface of fresh oyster shells, abalone, mussels, and clams. It also makes up the outer layer of the pearl. It’s very smooth, lustrous, and beautiful.
Pupa Gilbert, a physics professor at the University of Wisconsin, used polarized x-ray beams and nanoscale imaging at the Advanced Light Source (ALS) facility, at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) to study the structure of nacre. He commented that Nacre’s …
“…composed of thin layers of crystalline aragonite tablets separated by even thinner layers of organic material. … The aragonite tablet crystals in nacre are misoriented with respect to each other. This unique structural arrangement … could play a role in nacre’s remarkable resistance to fracture.” (Yarris, 2008)
Aragonite, also composed of calcium carbonate, has an orthorhombic crystal lattice of acicular crystals, meaning that it forms massed branches of needle-like crystals.G13
Contributing Author – Jeffery Bacon