Nacre in numbers
| Property | Pearl |
|---|---|
| Chemistry | CaCO3 (calcium carbonate) |
| Mohs hardness | 2.5 to 3.0 |
| Refractive index | 1.52 to 1.69 |
| Specific gravity | 2.60 to 2.85 |
From GIA's pearl gem description.
Why nacre glows
Light passes into the thin layers and reflects back from many of them, which is where luster and orient come from. GIA defines luster as the intensity of light reflected from the surface, and orient as iridescent rainbow colors shimmering on or just below it.
Why nacre is soft
At 2.5 to 3.0 on the Mohs scale, pearls scratch easily. GIA recommends storing them apart from other gems and metal jewelry. See how to clean and care for pearls.
Nacre thickness
A bead-nucleated cultured pearl is a shell bead with a coat of nacre. GIA checks that the nacre is not too thin or damaged, so the pearl will last. Tissue-nucleated freshwater pearls are almost solid nacre, and South Sea pearls have exceptionally thick nacre (GIA).
Non-nacreous pearls
Not every pearl is nacre. Conch, melo, quahog and some clam pearls are non-nacreous, with a porcelain-like surface and sometimes flame patterns (GIA).
Nacre and mother-of-pearl
The shiny lining inside a pearl oyster's shell is nacre too, and it is called mother-of-pearl. See mother-of-pearl.
