_Stream gravels_ (_placers_)—deposits of heavier and more durable than average minerals that have been eroded out of the original rock. Often tourmaline, beryl, and many other gem-quality minerals have eroded out of the original rock in which they formed and have moved and been concentrated locally by water in streams. Sapphires in Judith Basin County, Montana, were first found when the gravels were worked for gold from 1895 to 1930. _Metamorphic rocks_—rocks that have been altered by great heat, pressure, or both. Garnet, for example, is commonly found as crystals in gneiss and mica schist. [Illustration: Quartz with phantoms, Brazil.] [Illustration: Aquamarine, Brazil.] Mineral gemstones Hardness and specific gravity are two of the major characteristics of gemstones. Hardness of a gemstone is its resistance to scratching and may be described relative to a standard scale of 10 minerals known as the Mohs scale. F. Mohs, an Austrian mineralogist, developed this scale in 1822. According to Mohs’ scale, the hardness of— Talc is 1 Gypsum is 2 Calcite is 3 Fluorite is 4 Apatite is 5 Feldspar is 6 Quartz is 7 Topaz is 8 Sapphire is 9 Diamond is 10 Specific gravity is the number of times heavier a gemstone of any volume is than an equal volume of water; in other words, it is the ratio of the density of the gemstone to the density of water. _The 16 mineral gemstone groups listed below are highly prized for their beauty, durability, and rarity_: Beryl (hardness: 7.5-8 Mohs) Beryllium aluminum silicate Specific gravity: 2.63-2.91 _Emerald_: Intense green or bluish green _Aquamarine_: Greenish blue or light blue _Morganite_: Pink, purple pink, or peach _Heliodore_: Golden yellow to golden green _Red beryl_: Raspberry red _Goshenite_: Colorless, greenish yellow, yellow green, brownish Chrysoberyl (hardness: 8.5 Mohs) Beryllium aluminum oxide Specific gravity: 3.68-3.78 _Chrysoberyl_: transparent yellowish green to greenish yellow and pale brown _Alexandrite_: red in incandescent light and green in daylight _Cat’s eye_: usually yellowish or greenish Corundum (hardness: 9 Mohs) Aluminum oxide Specific gravity: 3.96-4.05 _Ruby_: Intense red _Sapphire_: Blue Diamond (hardness: 10 Mohs) Carbon Specific gravity: 3.51 Colorless to faint yellowish tinge, also variable Feldspar (hardness: 6-6.5 Mohs) Two distinctly different alkali alumino silicates: the Plagioclase and the Alkali Feldspar Series Specific gravity: 2.55-2.76 Plagioclase Series— _Labradorite_: Colorful, iridescent, also transparent stones in yellow, orange, red, and green _Sunstone_: Gold spangles from inclusions of hematite _Peristerite_: Blue white iridescence