Identifying a broken pearl, like the Tahitian shown here, is usually easy, since 99 percent of all Tahitian bead nuclei are composed of shell, which is recognized by its straight growth layers. But the broken bead of this pearl has a random, gritty structure, casting doubt on its origin. Another indication that the pearl might be imitation is the lack of a conchiolin layer, the organic layer between the bead nucleus and the nacre. There are, however, other means of identification.
A refractometer indicates that the refractive index (RI) of the top layer of the pearl covers a very large range, typical of calcite, a key component of nacre. (Nacre shows what’s called a birefringence “blink” on the refractometer. No pearl imitation can do this. Finding a carbonate blink is a fairly easy test for the desktop gemologist familiar with the equipment.)
Reexamination of the pearl under high magnification reveals conchiolin after all. This, along with the carbonate RI blink, proves that the specimen is a cultured pearl, despite its lack of a shell-bead nucleus.
Karin Hurwit, pearl expert formerly with the GIA Gem Trade Laboratory, cites yet another identification method: “Check first the top dark layer. Does it show suture lines? If so, you have a cultured pearl regardless of the identity of the nucleus.” Suture lines indicate nacre.
While few of us encounter a broken pearl to see what’s inside, Hurwit says many substances can be used as bead nuclei. “The fact that you do not see a conchiolin layer does not rule out that it is not a (cultured) pearl,” she says. “It really means that the growth was so smooth that less organic matter was deposited. The finest pearls I have examined showed very little conchiolin deposit.”
Gina Latendresse, pearl expert from American Pearl Co., Nashville, Tenn., also has seen other nuclei. “The composite nuclei hit the pearl-farming world with a large degree of usage about 1995,” she says. “That was the time when Chinese saltwater farming began cultivating in large quantities.”
The Chinese needed to find an alternative to American shell nuclei, which Latendresse says is the best and most reliable nuclei in the world, but also the most expensive. “So I saw dolomite, composites, giant clam, and other oyster/mussel species carved into nuclei for pearl culture,” she says. “I believe that the Chinese have been carving nuclei from their own freshwater mussels and mixing them in with the American nuclei.”
Tahitian pearl farmers were also looking at profit margins. “Because the Tahitians relied on the Japanese for their production of the nuclei, it was not uncommon at that time to experiment with other nuclei to lower costs. It’s not surprising to see this,” says Latendresse.