Pearl
Layered nacre grown around an irritant. Perfume, hairspray and acid skin dull it; a soft cloth after wear keeps it.
Hardness — 2.5 to 4.5 Care — soft cloth; keep from perfume Source — saltwater and freshwater farms
Pearl & shell
Nacre — aragonite laid down by a living animal. Soft by mineral standards and treated accordingly: last thing on, first thing off, wiped after wear.
Abalone
Shell, not stone. Layered nacre that reads green, blue and rose depending on the angle. Soft, so it is set protected and kept out of ultrasonic cleaners.
Hardness — 3.5 to 4 Care — soft cloth only Source — Pacific coast
Mother-of-pearl
The nacre lining of the shell, cut flat and quiet. Polished, it reads almost as light rather than as colour.
Hardness — 3.5 to 4 Care — soft cloth only Source — Pacific coast