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Ruby crystal on calcite
photo by Rob Lavinsky |
Gems have been prized for thousands of years, for their
color, luster, durability, hardness and their high
intrinsic value per
volume. They are found in all kinds of
geologic environments of all ages from thousands of million years to recent
times. Gemstones are also found in all
kinds of rocks ranging from igneous to metamorphic to sedimentary and are
classified according to the environment where they were formed. The
deposits are rare because of the
geological conditions needed to form them are exceptional. For example, in order for the Emerald to form
the beryllium from which it is made has to come in contact with chromium or
vanadium. These elements travel in
entirely different geochemical circles that are the exception rather than the
rule.
It is important to realize that like any other
commodity,
gems are subject to the laws of supply and demand. Even though the demand for gems remains about
the same the supplies are becoming depleted.
Many of the gems especially the colored variety are mined in
Third
World countries by artesianal methods, i.e. by hand. In any of these countries the discovery of
the new deposit often leads to a local prosperity, but many of these stones
become involved in the
illegal trade that is being used to finance
terrorism.
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Sapphire crystal
Photo by Rob Lavinsky |
These deposits of gemstones are volcanic in origin and are
formed deep within the Earth’s crust.
Gemstones from this origin have formed over 100 Km below the surface and
are brought to the surface as the result of magma intrusions or lava
flows. Diamonds are one type of this
deposit that has formed from 100 to 150 Km below the surface. These gemstones are found in potassium rich
stone called “kimberlite” that is brought to the surface in an explosive eruption
that proceeds at near supersonic speeds.
There seems to be a rule known as
Clifford's rule that these eruptions take place in Achaean terranes or come up
through younger terranes overlying Achaean terrenes. Kimberlite is quite rich in peridot and appears
to be full of xenoliths that are torn off the walls of the magma channel. A cross-sectional view of the kimberlite is
like looking at a carrot. The stone
itself is quite soft and easily eroded away with the diamonds being carried
away with the rest of the erosion products.
Diamonds are not the only product that is found in a
magmatic deposit so are others stones such as sapphires and zircons. Sapphires are also formed under some very
specific geological conditions the most important one is a magmatic stone that
is low in primary silicates such as basalt.
The sapphires form as crystals in the basalt and they're released as
products of erosion. Although they form
in the basalt they are rarely mined directly.
Zircons are another example of a gemstone that is found in a magmatic
deposit. Once again these gemstones are
not mined directly from the stone that are found as the products of
erosion. In geology zircons are thought
to be forever and the oldest dated minerals in the world are some zircons that
were found in
Western Australia
that were dated at 4.5 billion years old.
Zircons are used by
geochemists worldwide for dating formations of
different ages, and are highly effective in this use.
Some other gemstone minerals that are found in magmatic
deposits include:
Peridot, gem quality olivine usually found in mantle
xenoliths.
Labradorite, found as gem quality grains in some basalts.
Zircon, found as crystals in granite, basalt and the rarer
carbonatites.
Apatite, found as crystals in carbonatites.
Garnets, found as crystals in basalts as xenocrysts and
granite.
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Emerald crystal on calcite
Photo by Eva Krocher |
Pegmatites are hydrothermal deposits contain more different
types of gemstones in any other rock.
These are formed from the volatile rich fluids that are found in
pegmatites when they are formed. In some
geological circles pegmatites are called “Giant granite” because they have the
same minerals as granite except the crystals of the individual minerals are
much larger. There are two different
kinds of pegmatites simple and complex; most of the valuable minerals are found
in complex pegmatites. Although many of
these minerals could be classified as gems other minerals found in pegmatites
are the ores for rare metals such as niobium or rare earths.
Depending on the fluids present when the pegmatite is
solidifying different gemstones are deposited.
If the fluid is rich in fluorine topaz will be deposited. A beryllium rich fluid will produce the
mineral beryl in its many forms ranging from aquamarine, emerald, morganite, or
heliodor. Lithium rich fluid is another
possibility depositing spodumene or other lithium rich minerals. Pollucite is deposited from a cesium rich
fluid. In some cases the fluid is rich
in several different elements producing “Tourmaline” a gemstone that is
actually rarer them diamonds. A
manganese rich fluid will cause spessertine garnet to be deposited.
There are specific zones in a pegmatite where gemstones are
found. Many of them are found in
cavities called “Vugs.” The crystals formed
in this environment are usually clear and well formed. A deposit of this nature containing red and
green tourmaline was discovered in the 1970s at Newry,
Maine that was worth more then $8 million
from a single vug as large as a bedroom.
There are other hydrothermal environments that are also
capable of producing gemstones ranging from volcanic rocks to sedimentary
rocks. This is where one is likely to
encounter agates, petrified wood, opal, amethyst as well as a whole plethora of
gems. Turquoise is another mineral
deposited by hydrothermal waters. It is
copper phosphate and its beautiful sky blue botryoidal masses have been admired
for over two thousand years.
The only gem minerals normally found in metamorphic rocks are garnet,
zoisite (variety tanzanite), rubies and emeralds. Rubies are the red variety of the mineral
corundum that is aluminum oxide. These
gems are found in cordierite rich gneiss as well as marble. Sometimes rubies are carried up from the
depths of the earth as xenocrysts in basalt.
Most of the world's emeralds are found in low grade carbonaceous schist
in
Colombia.
Other
gemstones that occur in metamorphic rocks are iolite (the gem-quality
lilac-purple variety of cordierite), titanite and kyanite.
Jade is another mineral that is
found in high-pressure; low-pressure blue schist metamorphic rocks. There are two entirely different minerals
that are recognized as jade in the trade one of these as jadeite a compact
variety of the mineral pyroxene, and the other is nephrite a compact variety of
the mineral that is derived from tremolite.
Although these minerals are composed of common minerals it takes a very
special environment for them to form.
Sedimentary deposits:
By far the most valuable gemstone
found in sedimentary rock is precious opal.
The largest deposits of this mineral in the world are found in central
Australia. Other deposits are found in
the United States and Mexico. Most opal
is found as a replacement for fossils.
Opal also occurs as replacement beds or blebs in sedimentary rock. A good piece of opal showing good colors can
be as expensive as a diamond.
Placer deposits:
In some placer deposits gemstones
are relatively abundant particularly in areas where the local rocks are known
to produce gemstones. This is especially
true in areas underlying by cordierite gneiss or marble. Another area is where gemstone bearing
pegmatites are abundant. Because of
their toughness and hardness gemstones that are eroded from their country rock
tend to remain intact preserving the best gemstones. The same rough and tough treatment these
stones go under as a result of natural processes particularly running water the
weaker and softer stones are broken up leaving behind the highest quality
gemstones. Most of the gemstones in the
world are recovered from placer deposits.
In some cases the work of recovery can be quite primitive; in others the
most modern technology and machinery is used.
References:
Mineralogical Association of
Canada, Editor Lee A. Groat, Geology Of Gem Deposits, Short Course Series,
Volume 37, Yukon Geological Survey, © 2007, Yellowknife, Northwest Territories
Gem Deposits, http://amonline.net.au/geoscience/earth/gem.htm
Geology of Gem Deposits, Mineralogical
Association of Canada, Editor Lee A. Groat, http://www.mineralogicalassociation.ca/doc/promo_SC37.pdf Volume 37 © 2007
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