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
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
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
Mohs' scale of hardness
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
Beryl(hardness: 7.5-8 Mohs)
The Beryl species is famous for its most notable member, emerald. But the
beryl family also includes aquamarine, morganite, golden beryl and
All of the beryls are aluminum beryllium silicates, and have a hardness of
7.5 to 8 on the Mohs scale.
The different varieties of beryl are distinguished by their color, but
they differ significantly in their transparency and clarity as well.
For example, where emerald tends to be heavily included, aquamarine and
golden beryl will usually have excellent transparency and clarity.
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
Chrysoberyl(hardness: 8.5 Mohs)
Chrysoberyl was was first discovered in 1789 and since then, it has become
one of the most important and valuable gemstone groups.
It was formerly referred to as 'chrysolite', a historical name essentially
describing any greenish-golden to olive colored gemstone.
The chrysoberyl group is perhaps most famous for its cat's eye gemstone
variety, but the rarest and most valuable chrysoberyl is alexandrite - a
highly desirable chromium-rich color change variety of chrysoberyl.
Beryllium aluminum oxide Specific gravity: 3.68-3.78
Chrysoberyl: transparent yellowish green to greenish yellow and
Alexandrite: red in incandescent light and green in daylight
Cat's eye: usually yellowish or greenish
Corundum(hardness: 9 Mohs)
The corundum species includes two of the most important colored gemstones:
ruby and sapphire.
Famous for their excellent hardness (9 on the Mohs scale) and rarity, ruby
and sapphire share both a common chemical composition (aluminum oxide) and
a crystal structure (trigonal).
The main difference is color: ruby is red corundum, while all other
corundum colors are classified as sapphire.
Aluminum oxide Specific gravity: 3.96-4.05
Ruby: Intense red
Diamond(hardness: 10 Mohs)
Carbon Specific gravity: 3.51
Colorless to faint yellowish tinge, also variable
Feldspar makes up nearly 60% of the earth's crust. Feldspar is a group of
minerals distinguished by the presence of aluminum and the silica ion in
There are two main subgroups of feldspar that produce gem-quality
the potassium feldspars and the plagioclases, a
series that ranges from calcium to sodium feldspars.
Among the well-known feldspar gemstones are moonstone, orthoclase,
amazonite, andesine, labradorite and sunstone. Amazonite, moonstone and
orthoclase are all potassium feldspars.
They have a hardness of 6 to 6.5 on the Mohs scale, and a vitreous luster.
Moonstone also exhibits a unique shimmer known as adularescence.
Labradorite, andesine and sunstone are plagioclases feldspars. Like the
potassium feldspars, they have a hardness of 6 to 6.5, but they tend to
have slightly higher refractive indices, ranking between beryl and quartz.
Two distinctly different alkali alumino silicates: the
Plagioclase and the Alkali Feldspar Series Specific gravity: 2.55-2.76
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
Moonstone: Colorless; also white to yellowish, and reddish to bluish
Garnet(hardness: 6.5-7.5 Mohs)
he garnets are a large group of differently colored minerals with similar
crystal structure and related chemical compositions.
Garnet has historically been highly regarded for its very good hardness
(7-7.5 on the Mohs scale) and great brilliance.
The garnet family includes the inexpensive red garnets, almandine and
pyrope, as well as some rarer varieties.
The premium garnets include the red-orange spessartite, the green
tsavorite and demantoid, and the purple-red rhodolite.
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
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
Rhodolite: Purplish red to red purple
Spessartine: Yellowish orange
Uvarovite: Emerald green
Jade(hardness: 6 Mohs)
Jade has been known and treasured for more than 7,000 years. But only in
1863 was it discovered that jade is actually not a single mineral.
What was traditionally called jade is in fact two separate and distinct
minerals: jadeite and nephrite. Nephrite is the more common form of jade. Nephrite ranges in color
from mid to dark green or grey-green, but it can also be white, yellowish
or reddish. Nephrite is slightly softer than jadeite; nephrite is 6-6.5 on
the Mohs scale, while jadeite is 6.5-7. They have quite different chemical
compositions as well: nephrite is a calcium magnesium iron silicate while
jadeite is a sodium aluminum silicate. The two minerals also have
different densities. Jadeite has a density of 3.30-3.38 while nephrite is less dense at
2.90-3.03. The two varieties of jade even have different crystal
structures. While jadeite's structure is an arrangement of grainy
crystals, nephrite is made up of fibrous crystals that interlock in a
matted texture. These densely packed and interwoven fibers are extremely
resistant to fracturing. So while jadeite is the denser and harder jade,
nephrite is actually the tougher of the two
Nephrite 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)
It has been mined from Afghanistan since the early 7th millennium BC,
usually forms in crystalline marble through the geological process of
contact metamorphism and due to its composition, it is technically defined
as a rock rather than a mineral. It is primarily composed of lazurite,
while the remaining composition is made up of sodalite, calcite, pyrite
and other various minor constituents.
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)
The opal species has three subgroups:
the precious opals,
the yellow-red fire opals,
the common opals.
The precious opals are distinguished by their unique play of color, a
display of rainbow-lie hues which changes with the angle of observation.
The fire opals, named for their orange color, usually show no play of
color. Most fire opals are milky and turbid, but especially fine qualities
are clear and transparent.
The common opals are opaque to translucent and display no play of color;
colors range from white to yellow to brown.
Opals are unusual in that they are amorphous -- having no crystal
structure -- and always contain water (from 3 to 30 percent).
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)
A tiny island known as Zabargard (off Egypt) is documented as the first
source of peridot, dating back as far as four thousand years.
Early Crusaders who visited St. John's Island (now called Zabargard)
introduced this gem to Europe upon return from battle.
Peridot is mentioned in many ancient references as chrysolite.
Magnesium iron silicate Specific gravity: 3.22-3.45
Olive to lime green
Quartz(hardness: 7 Mohs)
Quartz is the second most common mineral on earth, making up about 12% of
the earth's crust. Only feldspar is more common.
There are two different branches of the quartz family. Macrocrystalline quartz includes rock crystal, amethyst, citrine,
smoky quartz, rose quartz and tiger's eye. Macrocrystalline quartz, as the
name suggests, has large crystals which can be distinguished by the naked
eye. This type of quartz is mainly transparent to translucent, with a
The other type of quartz is known as cryptocrystalline quartz. It
has microscopically (or submicroscopically) small crystals and is usually
translucent to opaque, with a waxy to greasy or dull luster. This kind of
quartz has fibrous and granular subcategories. The fibrous varieties are
known under the name chalcedony, but this name covers a large variety of
stones, including agate, jasper, bloodstone, carnelian and chrysocolla
Silicon dioxide or silica Specific gravity: 2.65
Coarsely crystalline varieties of silica-
Rock crystal: Colorless
Citrine: Yellow to amber
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)
Spinel is an important group of gemstones that for centuries was mistaken
It can occur in a variety of colors and is one of the few
singly-refractive gemstones available today.
Spinel gems have excellent hardness and durability and are never treated
or enhanced. Fine red spinel is even rarer than red ruby and is often
considered more valuable.
Magnesium aluminum oxide Specific gravity: 3.58-4.06
Balas ruby: Red
Almandine spinel: Purple red
Sapphire spinel and ghanospinel: Blue
Chlorspinel : Green
Topaz(hardness: 8 Mohs)
Topaz is a very important group of gemstones that are popularly used for
commercial jewelry. Its history dates back over 2,000 years.
The topaz group is most famous for its attractive and vivid blue colored
variety, but topaz can actually occur in a variety of other colors - some
extremely rare, including red, pink, light-green, violet, yellow and
Topaz in its purest form is white or colorless and often used as a
substitute for diamond. Most topaz gemstones are heated or enhanced, but
many rare and valuable topaz gemstones are untreated.
Aluminum silicate fluoride hydroxide Specific gravity: 3.5-3.6
Wine yellow, pale blue, green, violet, or red
Tourmaline(hardness: 7-7.5 Mohs)
Tourmaline is one of the most versatile gemstone groups and includes a
number of species and gemstone varieties.
In mineralogy, the major tourmaline species based on chemical composition
include dravite, uvite, schorl, liddicoatite and elbaite.
Schorl is most common and is black to near-black in color. Tourmaline
gemstones are especially popular for mainstream jewelry.
Most tourmaline gemstones display distinctly different colors in the same
Like sapphire, tourmaline is often traded using descriptive names such as
'yellow tourmaline' or 'pink tourmaline' to market fancy-colored varieties
of tourmaline gemstones.
Complex aluminum borosilicate (Elbaite, Dravite, Uvite)
Specific gravity: 3.03-3.25
Brazilian emerald : Green
Dravite: Brown Indicolite: Dark blue
Rubellite: Pink to red
Turquoise(hardness: 5-6 Mohs)
Turquoise was among the first gems to be mined, and many historic sites
have been depleted, though some are still worked to this day.
These are all small-scale operations, often seasonal owing to the limited
scope and remoteness of the deposits.
Most are worked by hand with little or no mechanization.
Hydrous copper aluminum phosphate Specific gravity: 2.6-2.8
Sky blue; greenish blue
Zircon(hardness: 7.5 Mohs)
Zircon is too often confused with cubic zirconia, an artificial diamond
stimulant. However, zircon is a completely natural group of gemstones with
an ancient history that dates back over 4 billion years. The most famous
and valuable zircon is blue zircon, but its range of colors also include
colorless, green, dark-red, violet, yellow, brown and orange. Like
sapphire, zircon is often traded using color-specific names, such as blue
zircon, golden zircon and white zircon. There are also some trade names
used today for specific colored zircon.
Zirconium silicate Specific gravity: 4.6-4.7
Matura diamond: Colorless
Hyacinth: Yellow, orange, red, brown