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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
Natural gemstones
Diamond, corundum , beryl , topaz, and opal are generally classed as precious stones. All other gemstones are usually classed as semiprecious.
Geologic environment
Gemstones are not plentiful. Gemstones do not form "ore" deposits in the normal sense.
Gems, when present at all, tend to be scattered sparsely throughout a large body of rock or to have crystallized as small aggregates or fill veins and small cavities.
Even stream gravel concentrations tend to be small--a few stones in each of several bedrock cracks, potholes, or gravel lenses in a stream bed.
The average grade of the richest diamond kimberlite pipes in Africa is about 1 part diamond in 40 million parts "ore." Kimberlite, a plutonic igneous rock, ascends from a depth of at least 100 kilometers to form a diatreme . Moreover, because much diamond is not of gem quality, the average stone in an engagement ring is the product of the removal and processing of 200 to 400 million times its volume of rock.
Gemstones occur in most major geologic environments.
Each environment tends to have a characteristic suite of gem materials, but many kinds of gems occur in more than one environment. Most gemstones are found in igneous rocks and alluvial gravels, but sedimentary and metamorphic rocks may also contain gem materials.
Examples of geologic environments in which gemstones are found:
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.
Beryl Beryllium aluminum silicate Specific gravity: 2.63-2.91
Chrysoberyl Beryllium aluminum oxide Specific gravity: 3.68-3.78
Corundum Aluminum oxide Specific gravity: 3.96-4.05
Diamond Carbon Specific gravity: 3.51
Colorless to faint yellowish tinge, also variable
Feldspar Two distinctly different alkali alumino silicates: the Plagioclase and the Alkali Feldspar Series Specific gravity: 2.55-2.76
Garnet A group of silicate minerals Specific gravity: 3.5-4.3
Jade
White, deep green, creamy brown
White, leafy and blue green, emerald green, lavender, dark blue green and greenish black, deep emerald-green
Lapis lazuli A rock composed mainly of the mineral lazurite with variable amounts of pyrite and white calcite Specific gravity: 2.7-2.9
Deep blue, azure blue, greenish blue
Opal Hydrated silica Specific gravity: 1.98-2.25
Peridot Magnesium iron silicate Specific gravity: 3.22-3.45
Olive to lime green
Quartz Silicon dioxide or silica Specific gravity: 2.65
Spinel Magnesium aluminum oxide Specific gravity: 3.58-4.06
Topaz Aluminum silicate fluoride hydroxide Specific gravity: 3.5-3.6
Wine yellow, pale blue, green, violet, or red
Tourmaline Complex aluminum borosilicate Specific gravity: 3.03-3.25
Turquoise Hydrous copper aluminum phosphate Specific gravity: 2.6-2.8
Sky blue; greenish blue
Zircon Zirconium silicate Specific gravity: 4.6-4.7
Birthstones
Month of birth Gemstone Color January Garnet Dark red February Amethyst Purple March Aquamarine or Bloodstone Pale blue April Diamond or Rock Crystal Colorless May Emerald Bright green or Chrysoprase Pale green June Pearl or Moonstone Cream July Ruby or Carnelian Red August Peridot Pale green or Sardonyx Brown and white September Sapphire Pale to dark blue or Lapis Lazuli Deep blue October Opal or Tourmaline Variegated November Topaz or Citrine Yellow December Turquoise Sky blue
Organic gemstones
The four organic gemstone groups listed below are highly prized for their beauty and rarity. However, they are not as durable as gemstones from minerals:
Amber A mixture of hydrocarbons Specific gravity: 1.05-1.096
Hard fossil resin or sap of ancient pine trees. Usually amorphous . 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.
Coral Formed mainly of calcite or conchiolin, a horny organic substance Specific gravity: 2.60-2.70
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