One of the most intriguing entries in the Museum collection is a single valve, or half-shell, of a freshwater Cockscomb Pearl Mussel, Cristaria plicata. Most freshwater bivalves have a hefty, internal layer of nacre on their shells. But in this case, the inside of the valve is embellished with a two-inch tall figurine, a "cultural pearl," or “pearl image,” of a long-bearded Chinese Sage. In historic China, such items were bestowed or sold to pilgrims visiting sacred shrines. In the old days, a priest would carefully insert a tin image between the shell and the mantle (the outermost living layer of a bivalve), then return the mussel to a pool or protected pond. After a few months, the tin image would be coated with shell layers, as shown in the photos below.
With growth, bivalves naturally keep adding more shell material to the inside of their valves. This makes their shells gradually thicker, less prone to crushing. The shell-thickening process causes any materials placed between the valve and the mantle to be gradually coated with new layers of shell material, nacre, or mother-of-pearl, in this case. Natural and artificial pearls are the best known results of the normal shell-thickening process, and now you know about Pearl Images! (The valve originated from the Xinjiang Province, China, and was donated in 1994 to the Museum collection by Donald Dan.)