Pinctada
The genus of marine oysters that produces every commercially important saltwater pearl.
Pinctada fucata gives Akoya, Pinctada maxima gives South Sea in both silver-lipped and gold-lipped forms, and Pinctada margaritifera gives Tahitian.
The species sets the ceiling on size and the range of colour, because the pearl takes its tone from the shell's own nacre lining.
Freshwater pearls come from a different family altogether, the Unionidae mussels, which is why they behave so differently.
When you are buying
When a seller names a species rather than a marketing term, that is a good sign. It is a checkable claim, and it tells you the likely size and colour before you see the pearl.
Akoya Pearl
The classic small round white saltwater pearl from Japan and China, usually 5mm to 9mm. The pearl in a traditional single-strand necklace.
South Sea Pearl
The largest commercially grown pearl, from the gold-lipped or silver-lipped oyster. Typically 9mm to 16mm, white to deep gold.
Tahitian Pearl
A naturally dark saltwater pearl from the black-lipped oyster of French Polynesia. Grey to near-black, with green, aubergine or peacock overtones.
Saltwater Pearl
A pearl grown in an oyster in the sea. Rounder and rarer than freshwater, and considerably more expensive.
Pearl Farming
Growing pearls deliberately: nucleating molluscs, tending them in water for months or years, then harvesting.