lmost all minerals and gems are formed below
the earth's surface. Some are brought to the
surface through mining, some are brought to the
surface through earth processes like faulting,
folding, or volcanic eruptions.
How do minerals and gems form? Water
near the earth's surface interacts with minerals
and dissolves them. If the solution cools or
evaporates, minerals will precipitate. A
similar, familiar process is formation of salt
crystals by evaporation of seawater.
The mineral that forms is determined by what
the dissolved elements are. If the water has
interacted with silica-rich rocks (e.g.,
sandstone), silica-rich minerals will form like
Silica (SiO2)-based minerals:
amethyst (quartz); or opal. Of these, only opal is
non-crystalline (ordered blobs of gel less than a
micron in diameter).
If the water has interacted with copper-rich
rocks, copper minerals will form: Cu-bearing
minerals: malachite and azurite or turquoise.
Another way is the formation of gems by
hydrothermal processes. The solutions
involve rain water and/or water derived from
cooling magma . Gems crystallize from solution
when it encounters open spaces such as cracks. As
a result, 'veins' of minerals fill preexisting
cracks. Minerals such as emeralds or
tourmaline need unusual elements, like beryllium
(for emeralds) or boron (for tourmaline)
Metamorphic gems. Metamorphic rocks are
rocks changed by heat, pressure, and interaction
with solutions. This high temperature and
high pressure is needed to produce jadeite
(jade). In extremely rare cases, pressures
in metamorphic rocks may be high enough that
Some gems are formed in the mantle. The most
abundant upper mantle mineral is olivine.
Slabs of mantle material are brought to the
surface through tectonic activity and volcanism.
Gems that form deep in the mantle
: Rocks such as kimberlites come from quite
deep in the mantle and carry with them
diamonds. Diamonds are made from carbon.
The stable form of carbon at the earth's
surface is graphite. High pressures and
temperatures are required to convert graphite to
diamond. Thus, almost all diamonds formed about
100 miles below the earth's surface.
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