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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, aluminium, 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 cuttable 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. |