Tahitian pearls get their naturally grey to black body colors from their host oyster, the black-lipped saltwater oyster, also known as Pinctada margaritifera.
The building blocks of nacre begin with the pearl sac, which forms around an irritant or in our case, a mother of pearl bead nucleus, and begins secreting liquid nacre over it in concentric layers.
Nacre is made of Calcium Carbonate (known chemically as CaCo3) and conchiolin, which is like an organic “glue” holding the various layers together. Calcium carbonate consists of tiny aragonite crystalline platelets that are transparent and microns thick.
Once the mother of pearl bead nucleus has been inserted into the oyster’s gonad, a small, 1.0 mm piece of donor mantle tissue is placed on top of the bead.
Donor mantle tissue square is always chosen from a different oyster that displays gorgeous colors on its inside shell. Not only does the donor tissue help stimulate nacre formation, but also influences the pearl’s final coloration.
Donor mantle tissue being sliced into small 1.0 mm squares for grafting. These tiny pieces of tissue are thought to exert the greatest amount of influence on the resulting pearl’s color. Photo courtesy of tmperles.com
All the elements of pearl nucleation: mother of pearl bead nuclei, and tiny squares of donor mantle tissue (on the red block, lower left) are used to implant a pearl oyster and begin pearl formation.
The calcium carbonate starts out as a liquid mixture that is secreted continuously throughout the oyster’s life as it builds and extends the outer edge of its shell throughout its lifetime.
Nacre is created in the oyster’s mantle (the outer edge of the meaty interior of the animal); it can be completely transparent, or tinged with color, which is the case of Tahitian pearls. Other factors that go into pearl coloration are nutrients in the water, and natural chemicals found in the environment.