Natural pearls
GIA describes natural pearls as forming around a microscopic irritant in the bodies of certain mollusks. The FTC's definition says a natural pearl forms in response to an irritant or an abnormal physiological condition, with no human involvement. No one knows in advance which animal will make one, which is why natural pearls are rare.
Cultured pearls
People start the process on purpose. A nucleus, usually a sphere of shell, is placed inside the mollusk or its mantle, and the mollusk coats it with nacre (FTC). Some cultured pearls, including most freshwater pearls, start from a small piece of tissue instead of a bead, so they are almost solid nacre (GIA).
| Pearl | Mollusk | Pearls per animal | Time to grow |
|---|---|---|---|
| Akoya | Pinctada fucata oyster | Usually one at a time | 10 to 24 months |
| Tahitian | Pinctada margaritifera oyster | One | About 18 to 24 months |
| South Sea | Pinctada maxima oyster | One | Often 2 to 4 years |
| Freshwater | Hyriopsis mussels | Several in one cycle | Not stated by GIA |
All from GIA.
Do all oysters make pearls?
No. The pearls in jewelry come from particular pearl oysters and mussels. Some other mollusks make pearls that are not nacreous at all, such as the conch, melo and quahog, and these have a porcelain-like surface instead of a nacre glow (GIA).
How long does it take?
From about 10 months for some akoyas to 2 to 4 years for South Sea pearls. At Lake Kasumigaura in Japan, cultured pearls grow for about 3.5 to 4 years and build nacre about 3 mm thick (GIA).
What a pearl is made of
Calcium carbonate, plus organic material that binds the layers (FTC, GIA). Read what pearls are made of.
