Coral (hardness: 3.5-4 Mohs) Formed mainly of calcite (calcium carbonate) or conchiolin, a horny organic substance 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 semitranslucent 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. Jet (hardness: 2.5-4 Mohs) 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. Pearl (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. Buyer beware Inexperienced buyers must take whatever steps are needed to ensure that gems they intend to purchase are exactly what the seller purports them to be and that they are being offered at a fair market price. More and more synthetic gems—and inferior grades and cuts of natural gems—are being sold to unwary buyers by unscrupulous sellers. Since 1935, the mining of gemstones in the United States has been almost entirely a recreational activity of mineral collectors and hobbyists. In recent years, very few individuals have derived their entire income from gemstones mined by themselves. This is not to say that the proprietors of roadside rock shops buy all of their stock from others. Rock shops are abundant in areas of the United States that are rich in gem materials, and the shops tend to specialize in the local gem commodities, most of which the proprietors gather. Rather than doing the mining themselves, owners of land that has a deposit of gem-quality minerals sometimes charge hobbyists for the right to collect gemstones. For example, diamond in Arkansas, opal in Idaho, and agate in Oregon and Washington are mined by hobbyists under this “fee digging” arrangement.