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.