Tissue Nucleated
Grown around a scrap of mantle tissue instead of a bead, so the pearl is solid nacre throughout. The freshwater method.
A technician inserts a small piece of donor mantle tissue into the mussel. The tissue forms a pearl sac and the mussel fills it entirely with nacre.
With no bead inside, the pearl cannot be sliced open to reveal a core, and nacre thickness stops being a meaningful risk.
The trade-off is shape. Without a round bead to grow on, perfect roundness is far harder to achieve, which is why round freshwater pearls command a premium over oval and semi-round ones.
When you are buying
This is the strongest technical argument for freshwater pearls. A tissue-nucleated pearl cannot lose its nacre layer, because it is nacre all the way down.
Bead Nucleated
Grown around a round shell bead, so the pearl is a nacre shell over a core. The saltwater method, and how roundness is achieved.
Nacre
The material a pearl is made of: microscopic aragonite platelets glued in layers by conchiolin. Thickness determines both lustre and lifespan.
Freshwater Pearl
A pearl grown inside a mussel in a river or lake, rather than an oyster in the sea. Most pearl jewellery sold today is freshwater.
Keshi Pearl
A pearl that forms by accident with no nucleus at all, so it is solid nacre. Irregular in shape and unusually bright.