gemstone deposits

Gemstones and other deposits

geological environment
examples of geologic environments in which gemstones are found
gemstones characteristics
mohs scale of hardness
the 16 mineral gemstone groups
beryl  chrysoberyl  corundum  diamond
feldspar  garnet
lapis lazuli

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, forexample, is commonly found as crystals in gneiss and mica schist.

Mineral Gemstones Characteristics

 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 ofwater; in other words, it is the ratio of the density of the gemstone to the density of water.

The 16 mineral gemstone groups

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)

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)


Calcium magnesium silicate

Specific gravity: 2.9-3.1

 White, deep green, creamy brown


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

 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

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