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Natural Gemstones I* ^H U.S U.S. Department of the Interior U.S. Geological Survey Natural Gemstones i* ^H U.S. Department of the Interior U.S. Geological Survey Natural Gemstones Searching for gemstones in the United States is a popular recreational activity for collectors and hobbyists Cover: Quartz, Rhode Island. Inside covers: Amethyst crystals, Mexico. Natural gemstones The average grade of the richest diamond kimberlite pipes in Africa is about 1 part A natural gemstone is a mineral, stone, or diamond in 40 million parts "ore." organic matter that can be cut and polished Kimberlite, a plutonic igneous rock, as­ or otherwise treated for use as jewelry or cends from a depth of at least 100 kilome­ other ornament. A precious gemstone has ters (60 miles) to form a diatreme (narrow beauty, durability, and rarity, whereas a cone-shaped rock body or "pipe"). More­ semiprecious gemstone has only one or two over, because much diamond is not of gem of these qualities. A gem is a gemstone that quality, the average stone in an engagement has been cut and polished. ring is the product of the removal and pro­ cessing of 200 to 400 million times its Diamond, corundum (ruby and sapphire), volume of rock. beryl (emerald and aquamarine), topaz, and opal are generally classed as precious stones. All other gemstones are usually Gemstones occur in most major classed as semiprecious. geologic environments. A mineral is any naturally formed homoge­ Each environment tends to have a charac­ neous inorganic material. teristic suite of gem materials, but many kinds of gems occur in more than one envi­ A mineralogist is a person who studies the ronment. Most gemstones are found in formation, occurrence, properties, composi­ igneous rocks and alluvial gravels, but tion, and classification of minerals. sedimentary and metamorphic rocks may also contain gem materials. A gemologist is a person who has success­ fully completed recognized courses in Examples of geologic environments in gemology (the science and study of gem- which gemstones are found: stones) and has proven skills in identifying and evaluating gem materials. Pegmatite a coarse-grained intrusive igneous rock body, occurring as dikes (a A lapidary is a cutter, polisher, or engraver tabular-shaped body), lenses, or veins in the of precious stones. surrounding rock. Stream gravels (placers) deposits of Geologic environment heavier and more durable than average minerals that have been eroded out of the Gemstones are not plentiful. Gemstones do original rock. Often tourmaline, beryl, and not form "ore" deposits in the normal sense. many other gem-quality minerals have eroded out of the original rock in which Gems, when present at all, tend to be scat­ they formed and have moved and been tered sparsely throughout a large body of concentrated locally by water in streams. rock or to have crystallized as small aggre­ Sapphires in Judith Basin County, Mon­ gates or fill veins and small cavities. tana, were first found when the gravels were worked for gold from 1895 to 1930. Even stream gravel concentrations tend to be small a few stones in each of several Metamorphic rocks rocks that have been bedrock cracks, potholes, or gravel lenses altered by great heat, pressure, or both. in a stream bed. Garnet, for example, is commonly found as crystals in gneiss and mica schist. Quartz with phantoms, Brazil. Aquamarine, Brazil. Mineral gemstones Corundum (hardness: 9 Mohs) Aluminum oxide Hardness and specific gravity are two of the Specific gravity: 3.96-4.05 major characteristics of gemstones. Ruby: Intense red Hardness of a gemstone is its resistance to Sapphire: Blue scratching and may be described relative to a standard scale of 10 minerals known as Diamond (hardness: 10 Mohs) the Mohs scale. F. Mohs, an Austrian min­ Carbon eralogist, developed this scale in 1822. Specific gravity: 3.51 According to Mohs' scale, the hardness of Colorless to faint yellowish tinge, also Talc is 1 Feldspar is 6 variable Gypsum is 2 Quartz is 7 Calcite is 3 Topaz is 8 Feldspar (hardness: 6-6.5 Mohs) Fluorite is 4 Sapphire is 9 Two distinctly different alkali alumino Apatite is 5 Diamond is 10 silicates: the Plagioclase and the Alkali Feldspar Series Specific gravity is the number of times Specific gravity: 2.55-2.76 heavier a gemstone of any volume is than an equal volume of water; in other words, it Plagioclase Series is the ratio of the density of the gemstone to Labradorite: Colorful, iridescent, also the density of water. transparent stones in yellow, orange, red, and green The 16 mineral gemstone groups listed Sunstone: Gold spangles from inclusions below are highly prized for their beauty, of hematite durability, and rarity: Peristerite: Blue white iridescence Beryl (hardness: 7.5-8 Mohs) Alkali Feldspar Group Beryllium aluminum silicate Orthoclase: Pale yellow, flesh red Specific gravity: 2.63-2.91 Amazonite: Yellow green to greenish blue Moonstone: Colorless; also white to Emerald: Intense green or bluish green yellowish, and reddish to bluish gray Aquamarine: Greenish blue or light blue Morganite: Pink, purple pink, or peach Garnet (hardness: 6.5-7.5 Mohs) Heliodore: Golden yellow to golden green A group of silicate minerals Red beryl: Raspberry red Specific gravity: 3.5-4.3 Goshenite: Colorless, greenish yellow, yellow green, brownish Almandine: Orangy red to purplish red Almandine-spessartine: Reddish orange Chrysoberyl (hardness: 8.5 Mohs) Andradite: Yellowish green to orangy Beryllium aluminum oxide yellow to black Specific gravity: 3.68-3.78 Demantoid: Green to yellow green andradite Chrysoberyl: transparent yellowish green to Topazolite: Yellow to orangy yellow greenish yellow and pale brown Grossular: Colorless; also orange, pink, Alexandrite: red in incandescent light and yellow, and brown green in daylight Tsavorite: Green to yellowish green Cat's eye: usually yellowish or greenish Hessonite: Yellow orange to red Pyrope: Colorless; also pink to red Chrome pyrope: Orange red Pyrope-Almadine: Reddish orange to red lazurite with variable amounts of pyrite purple (brassy flecks) and white calcite Pyrope-Spessartine: Greenish yellow to Specific gravity: 2.7-2.9 purple Malaia: Yellowish to reddish orange Deep blue, azure blue, greenish blue (bluish to brown color with flecks of white and gold) Color-change garnet. Blue green in day­ light to purple red in incandescent light Opal (hardness: 5.5-6.5 Mohs) Rhodolite: Purplish red to red purple Hydrated silica Spessartine: Yellowish orange Specific gravity: 1.98-2.25 Uvarovite: Emerald green White opal: Opaque, porcelain-like white Jade (hardness: 6 Mohs) material; colors resemble flashes or speckles Nephrite Black opal: Flashes and speckles appear Calcium magnesium silicate against black background Specific gravity: 2.9-3.1 Water opal: A transparent, colorless opal is the background for brilliant flashes of White, deep green, creamy brown color Fire opal: Reddish or orange opal Jadeite Sodium aluminum silicate Peridot [Olivine] (hardness: 7 Mohs) Specific gravity: 3.1-3.5 Magnesium iron silicate Specific gravity: 3.22-3.45 White, leafy and blue green, emerald green, lavender, dark blue green and greenish Olive to lime green black, deep emerald-green Quartz (hardness: 7 Mohs) Lapis lazuli (hardness: 5-5.5 Mohs) Silicon dioxide or silica A rock composed mainly of the mineral Specific gravity: 2.65 Coarsely crystalline varieties of silica Rock crystal: Colorless Amethyst: Purple Citrine: Yellow to amber Morion: Black Smoky quartz or cairngorm: smoky gray to brown Rose quartz: Translucent pink Green quartz or praziolite: Green Cryptocrystalline varieties of silica Chalcedony and Jasper (variable) Agate: Bull's eye agate, Iris or fire agate, Onyx, Sardonyx. Bloodstone or heliotrope. Cornelian. Chrysoprase. Moss agate. Plasma. Prase. Sard. Jasper. Spinel (hardness: 8 Mohs) Diamond Star of Sierra Leone. Magnesium aluminum oxide Specific gravity: 3.58-4.06 Candelabra: white quartz, blue-capped red elbaite, and tan albite, California. Balas ruby: Red Indicolite: Dark blue Almandine spinel: Purple red Rubellite: Pink to red Rubicelle: Orange Siberite: Violet Sapphire spinel and ghanospinel: Blue Verdilite: Green Chlorspinel: Green Turquoise (hardness: 5-6 Mohs) Topaz (hardness: 8 Mohs) Hydrous copper aluminum phosphate Aluminum silicate fluoride hydroxide Specific gravity: 2.6-2.8 Specific gravity: 3.5-3.6 Sky blue; greenish blue Wine yellow, pale blue, green, violet, or red Zircon (hardness: 7.5 Mohs) Tourmaline (hardness: 7-7.5 Mohs) Zirconium silicate Complex aluminum borosilicate Specific gravity: 4.6-4.7 (Elbaite, Dravite, Uvite) Specific gravity: 3.03-3.25 Jargon: Variable Matura diamond: Colorless Achorite: Colorless Hyacinth: Yellow, orange, red, brown Brazilian emerald: Green Dravite: Brown The Hooker Emerald, Colombia. Birthstones Month Month of birth Gemstone Color of birth Gemstone Color January Garnet Dark red July Ruby (Corundum) Red February Amethyst (Quartz) Purple or Carnelian (Quartz) March Aquamarine (Beryl) Pale blue or Bloodstone August Peridot Pale green (Quartz) or Sardonyx Brown April Diamond or Colorless (Quartz) and white Rock Crystal September Sapphire Pale to dark (Quartz) (Corundum) blue May Emerald (Beryl) Bright green or Lapis Lazuli Deep blue or Chrysoprase Pale green October Opal or Tourmaline Variegated (Quartz) November Topaz or Citrine Yellow June Pearl or Moonstone Cream (Quartz) (Feldspar) December Turquoise Sky blue Organic gemstones This compact velvet-black coal takes a good polish and is often cut into beads, The four organic gemstone groups listed bracelets, and a wide range of decorative below are highly prized for their beauty and and useful objects. rarity. However, they are not as durable as gemstones from minerals: Pearl (hardness: 2.5-4.5 Mohs) Formed within a mollusk, such as an oyster, Amber (hardness: 2-2.5 Mohs) that deposits a substance called nacre A mixture of hydrocarbons around an irritant that entered the organism Specific gravity: 1.05-1.096 Specific gravity: 2.71 Hard fossil resin or sap of ancient pine Pearl-bearing mollusks are found in both trees.
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