Pearl Sac
The pocket of tissue that forms around an implant inside a mollusc and secretes nacre onto it.
When mantle tissue is inserted, the mollusc's cells grow around the implant and close into a sac.
That sac is what actually makes the pearl, laying down nacre layer by layer for as long as the animal is healthy.
In freshwater mussels the sac often keeps working after a pearl is harvested, which is how a second pearl can be grown in the same pocket.
Nucleation
The operation that starts a cultured pearl: inserting a bead, a piece of mantle tissue, or both.
Tissue Nucleated
Grown around a scrap of mantle tissue instead of a bead, so the pearl is solid nacre throughout. The freshwater method.
Keshi Pearl
A pearl that forms by accident with no nucleus at all, so it is solid nacre. Irregular in shape and unusually bright.
Nacre
The material a pearl is made of: microscopic aragonite platelets glued in layers by conchiolin. Thickness determines both lustre and lifespan.
Pearl Farming
Growing pearls deliberately: nucleating molluscs, tending them in water for months or years, then harvesting.