There are 4 organic gemstone groups:
A mixture of hydrocarbons
Specific gravity: 1.05-1.096
Hard fossil resin or sap of ancient pine trees. Usually amorphous (lacks crystalline structure). Sometimes mined, sometimes gathered on seashores.
Varies from transparent to semitransparent and generally from light yellow to dark brown, but can be orange, red, whitish, greenish-brown, blue, or violet. Can be dyed in any color.
Takes a fine polish. Used mainly in making beads or other ornaments.
Formed mainly of calcite or conchiolin
Specific gravity: 2.60-2.70
Each coral polyp, a tiny marine animal that lives in enormous colonies, extracts calcium carbonate from the sea and exudes it to build a protective home around and above itself. Each generation of polyps dies in its protective home and each succeeding generation builds on top of its predecessor.
Gem coral ranges from semi translucent to opaque and occurs in white, pink, orange, red, blue, violet, golden, and black. The black and golden corals are largely horny organic substances, not calcium carbonate.
The finest coral is used to make figurines, cameos, carvings, and beads.
Carbon plus various hydrocarbon compounds
Specific gravity: 1.30-1.32
This compact velvet-black coal takes a good polish and is often cut into beads, bracelets, and a wide range of decorative and useful objects.
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.
All info obtained from:
The U.S. Geological Survey
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