Natural Gemstones

Part 1

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

A natural gemstone is a mineral, stone, or organic matter that can be cut and polished or otherwise treated for use as jewelry or other ornament. A precious gemstone has beauty, durability, and rarity, whereas a semiprecious gemstone has only one or two of these qualities. A gem is a gemstone that has been cut and polished.

Diamond, corundum (ruby and sapphire), beryl (emerald and aquamarine), topaz, and opal are generally classed as precious stones. All other gemstones are usually classed as semiprecious.

A mineral is any naturally formed homogeneous inorganic material.

A mineralogist is a person who studies the formation, occurrence, properties, composition, and classification of minerals.

A gemologist is a person who has successfully completed recognized courses in gemology (the science and study of gemstones) and has proven skills in identifying and evaluating gem materials.

A lapidary is a cutter, polisher, or engraver of precious stones.

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 (60 miles) to form a diatreme (narrow cone-shaped rock body or “pipe”). 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:

Pegmatite—a coarse-grained intrusive igneous rock body, occurring as dikes (a tabular-shaped body), lenses, or veins in the surrounding rock.

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.

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:

Part 2

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

Alkali Feldspar Group— Orthoclase: Pale yellow, flesh red Amazonite: Yellow green to greenish blue Moonstone: Colorless; also white to yellowish, and reddish to bluish gray

Garnet (hardness: 6.5-7.5 Mohs) A group of silicate minerals Specific gravity: 3.5-4.3

Almandine: Orangy red to purplish red Almandine-spessartine: Reddish orange Andradite: Yellowish green to orangy yellow to black Demantoid: Green to yellow green andradite Topazolite: Yellow to orangy yellow Grossular: Colorless; also orange, pink, yellow, and brown Tsavorite: Green to yellowish green Hessonite: Yellow orange to red Pyrope: Colorless; also pink to red Chrome pyrope: Orange red Pyrope-Almadine: Reddish orange to red purple Pyrope-Spessartine: Greenish yellow to purple Malaia: Yellowish to reddish orange to brown Color-change garnet: Blue green in daylight to purple red in incandescent light Rhodolite: Purplish red to red purple Spessartine: Yellowish orange Uvarovite: Emerald green

Jade (hardness: 6 Mohs)

Nephrite Calcium magnesium silicate Specific gravity: 2.9-3.1

White, deep green, creamy brown

Jadeite Sodium aluminum silicate Specific gravity: 3.1-3.5

White, leafy and blue green, emerald green, lavender, dark blue green and greenish black, deep emerald-green

Lapis lazuli (hardness: 5-5.5 Mohs) A rock composed mainly of the mineral lazurite with variable amounts of pyrite (brassy flecks) and white calcite Specific gravity: 2.7-2.9

Deep blue, azure blue, greenish blue (bluish color with flecks of white and gold)

Opal (hardness: 5.5-6.5 Mohs) Hydrated silica Specific gravity: 1.98-2.25

White opal: Opaque, porcelain-like white material; colors resemble flashes or speckles Black opal: Flashes and speckles appear against black background Water opal: A transparent, colorless opal is the background for brilliant flashes of color Fire opal: Reddish or orange opal

Peridot [Olivine] (hardness: 7 Mohs) Magnesium iron silicate Specific gravity: 3.22-3.45

Olive to lime green

Quartz (hardness: 7 Mohs) Silicon dioxide or silica 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. Carnelian. Chrysoprase. Moss agate. Plasma. Prase. Sard. Jasper.

Spinel (hardness: 8 Mohs) Magnesium aluminum oxide Specific gravity: 3.58-4.06

Part 3

Balas ruby: Red Almandine spinel: Purple red Rubicelle: Orange Sapphire spinel and ghanospinel: Blue Chlorspinel: Green

Topaz (hardness: 8 Mohs) Aluminum silicate fluoride hydroxide Specific gravity: 3.5-3.6

Wine yellow, pale blue, green, violet, or red

Tourmaline (hardness: 7-7.5 Mohs) Complex aluminum borosilicate (Elbaite, Dravite, Uvite) Specific gravity: 3.03-3.25

Achorite: Colorless Brazilian emerald: Green Dravite: Brown Indicolite: Dark blue Rubellite: Pink to red Siberite: Violet Verdilite: Green

Turquoise (hardness: 5-6 Mohs) Hydrous copper aluminum phosphate Specific gravity: 2.6-2.8

Sky blue; greenish blue

Zircon (hardness: 7.5 Mohs) Zirconium silicate Specific gravity: 4.6-4.7

Jargon: Variable Matura diamond: Colorless Hyacinth: Yellow, orange, red, brown

Birthstones

Month of birth Gemstone Color January Garnet Dark red February Amethyst (Quartz) Purple March Aquamarine (Beryl) or Bloodstone Pale blue (Quartz) April Diamond or Rock Crystal (Quartz) Colorless May Emerald (Beryl) Bright green or Chrysoprase (Quartz) Pale green June Pearl or Moonstone (Feldspar) Cream July Ruby (Corundum) or Carnelian Red (Quartz) August Peridot Pale green or Sardonyx (Quartz) Brown and white September Sapphire (Corundum) Pale to dark blue or Lapis Lazuli Deep blue October Opal or Tourmaline Variegated November Topaz or Citrine (Quartz) 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 (hardness: 2-2.5 Mohs) 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.

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.

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