 The
Garnet group
of minerals show crystals with a habit of rhombic
dodecahedrons and trapezohedrons. They are
nesosilicates with the same general formula,
A3B2(SiO4)3. The chemical elements in garnet include
calcium, magnesium, aluminum, iron2+, iron3+,
chromium, manganese, and titanium. Garnets show no
cleavage and a dodecahedral parting. Fracture is
conchoidal to uneven; some varieties are very tough
and are valuable for abrasive purposes. Hardness is
6.5 - 7.5, specific gravity is 3.1 - 4.3, luster is
vitreous to resinous, and they can be transparent to
opaque.
The name "garnet" comes from the Latin granatus, a
grain possibly in reference to malum garanatum
(pomegranate) a plant with red seeds similar in shape,
size and color to some garnet crystals.
There is a misconception that garnets are only a red
gem but in fact they come in a variety of colors
including purple, red, orange, yellow, green, brown,
black, or colorless. The lack of a blue garnet was
remedied in 1990's following the discovery of
color-change blue to red/pink material in Bekily,
Madagascar but these stones are very rare.
Color-change garnets are by far the rarest garnets
except uvarovite, which does not come in cutable
sizes. In daylight, their color can be shades of
green, beige, brown, gray and rarely blue, to a
reddish or purplish/pink color in incandescent light.
By composition, these garnets are a mix of spessartine
and pyrope, as are Malaya garnets. The color change of
these new garnets is often more intense and more
dramatic than the color change of top quality
Alexandrite which is frequently disappointing, but
still sells for many thousands of dollars (US) per
carat. It is expected that blue color-change garnets
will match Alexandrite prices or even exceed them as
the color change is often better and these garnets are
much rarer. The blue color-change type is mainly
caused by relatively high amounts of vanadium (about 1
wt.% V2O3).
Six common varieties of garnet are recognized based on
their chemical composition. They are pyrope, almandine
or carbuncle, spessartite, grossularite (varieties of
which are hessonite or cinnamon-stone and tsavorite),
uvarovite and andradite. The garnets make up two solid
solution series; 1. pyrope-almandine-spessarite and 2.
uvarovite-grossularite-andradite.
Garnet is the birthstone for January, and has been
used since the Bronze Age. |
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