The lithium pegmatites may contain a score of minerals in which
lithium is either a major or minor constituent.
Of these, spodumene
is most frequently found, usually as chalky white crystals but
occasionally as gem material of rare beauty. At Pala, California,
Minas Gerais, Brazil, and Madagascar, gem spod-umene of a delicate
lilac-rose to amethystine-pink has been found.
These transparent crystals of great beauty and value display to a
marked degree the property of dichroism;
that is, the intensity and quality of the color depends on the
direction in which light passes through the crystal.
This lovely gem has been given the name kunzite,
for G. F. Kunz, a noted American gemmologist.
As the variety hiddenite, spodumene
has a striking emerald-green color. Hiddenite was discovered as a
result of the overturning of a tree on a farm near Stony Point, North
Carolina, by W. E. Hidden, for whom the gem variety was later named.
The spodumene gems are rare and of exceptionally
lovely color, but the pronounced prismatic cleavage makes them
difficult to cut and polish, and the relatively low hardness makes
them impractical in a ring mount.
Beryl, found in giant crystals firmly embedded
in pegmatites, is usually a yellow-green to blue-green color,
but the smaller flawless gem crystals are found in several colors to
which variety names are given. They characteristically occur in
well-formed hexagonal prisms commonly with brilliant crystal faces,
but sometimes are etched and corroded by natural solutions, giving
the crystals a fluted appearance.
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The best-known variety is aquamarine,
the transparent blue to sea-green gem.
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The name heliodor is given to lovely
golden-yellow beryl from Southwest Africa, but similar-colored stones
from elsewhere are called golden beryl.
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Morganite is a pale pink to rose-red
variety occurring in tabular crystals. It is believed that small
amounts of cesium are responsible for the tabular habit and small
amounts of manganese produce the pink color.
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Although some gem beryls from pegmatites are green, they never have
the deep green color of emerald.
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Emerald is the chrome green variety
Although chrysoberyl
is found in several types of deposits, some of the finest examples of
this gem material have come from pegmatites.
The name comes from the Greek chrysos, meaning golden, but this is
misleading, for chrysoberyl is most commonly a yellowish green
and frequently other shades from nearly white to deep emerald-green.
Alexandrite is the name given to a
variety of chrysoberyl that has the remarkable property of appearing emerald-green
by daylight and red by artificial light.
This stone was discovered in Takovaya, in the Ural Mountains in 1833
and was named alexandrite after the Czarevitch, later Czar Alexander II.
Some chrysoberyl, because of the presence of parallel needle-like
inclusions, shows, when polished as a cabochon, a band of light that
moves as the stone is turned.
This property led to the popular name of "cat's eye"
for such gems, and the term chatoyancy for the effect. "Cat's
eye" chrysoberyl is not to be confused with "tiger's
eye," a variety of quartz with a golden brown color and a
fibrous structure that produces a very prominent moving band of light
on a properly polished surface.
Cat's eye is actually a more valuable and less common stone
than tiger's eye. Chrysoberyl is also called cymophane,
from Greek kyma, meaning a wave, in allusion to the pale opalescence
that forms the cat's-eye effect.
Alexandrite, with its dramatic color change, described as "an
emerald by day and a ruby by night" is a valuable gem, but the
color change has been successfully imitated in synthetic spinel so
the would-be buyer of such stones should beware.
Not a year passes that mineralogists do not discover several hitherto
unknown mineral species. But as the science moves forward, discovery
shifts from the obvious to the more obscure minerals. Today, it is
not unusual for the description of a new mincral to be based on such
a small amount of material that practically all of it is consumed in
making the chemical analysis. It is thus remarkable when a new
mineral is found in large quantity, and even more remarkable when it
is found in large crystals of gem quality.