(hardness: 2.5-4.5 Mohs)

Formed within a mollusk, such as an oyster, that deposits a substance called nacre around an irritant that entered the organism
 
Specific gravity: 2.71

Pearl-bearing mollusks are found in both salt and fresh water. Salt-water pearls of gem quality are usually preferred for jewelry; they are produced almost entirely by the mollusk Pinctada. Fresh-water pearls are produced by various clams and mussels.
Natural pearls come in various shapes: round, pear, drop, egg, and others. They also come in various colors, such as white, cream, light rose, cream rose, black, gray, bronze, blue, dark blue, blue green, red, purple, yellow, and violet.