Crystal system: Cubic
Chemical formula/composition: A group of related silicate minerals. They have
a similar crystal structure, and some members are part of an isomorphic
series. Isomorphic meaning same arrangement of atoms, but
different chemical elements, for example, almandine (Fe3Al2(SiO4)3)
and pyrope (Mg3Al2(SiO4)3) form such a series. In this case, iron, Fe, can
substitute for Mg, magnesium, and the intermediate amounts of these
elements create intermediate composition minerals between the almandine
and pyrope end members
Crystal habit: Occurs mainly in dodecahedral forms, well-formed
crystals are common. Dodecahedron and trapezohedron
Hardness: Variable dependent on species, 6.5-7.5. Some
members of the garnet group are a little soft to wear in rings.
Almandine (7-7.5), Pyrope (7-7.5), Andradite
(6.5-7), Grossular (7-7.5), Spessartine (7-7.5), Uvarovite (7.5).
Specific gravity:
Luster: vitreous
Toughness: good
Cleavage: None
Optics: Isotropic
Color: Many colors, mostly green,
orange, red, and brown. Many different shades
Other: U.V. fluorescence: commonly
nonfluorescent
Varieties: Several
gem varieties, most species have gemstone
possibilities.
Localities: Worldwide
Common simulants
None, but colored glass may be falsely
substituted.
Spinel has very similar properties.
Synthetics: Synthetics are known
but rare. Synthetics such as GGG and YAG were first made for high
tech fields, but were found to be good diamond simulants. Green
YAG or GGG might be used to substitute for demantoid garnet.
The garnet group is one of the most chemically diverse, and
garnets
are found in many different types of rocks, most notably metamorphic,
but
also in igneous rocks such a granite and kimberlite. Gems are
produced from almost every variety, and come in a diverse number
of highly attractive colors. The color most often associated with
garnet is red, but they occur in many colors, green, orange, purple,
brown and are found in many shades of these colors. Good
gem quality crystals can be found that are relatively inclusion
free. Though garnets of 10 pounds or more in weight have been
found with frequency, usually gem quality stones are not very
large. Star garnets with 4 rayed stars and color-changing
garnets similar in behavior to alexandrite are known, but are rare.
Garnet has excellent luster and nice color, and certain
species such as demantoid and Tsavorite are in high demand.
Types of garnet
1) Pyrope. Bohemian garnets (pyrope) are perhaps
the most frequent found in jewelry. Pyrope is usually a strong
wine red, but can be purple to brown. Some contains chromium (Cr)
such as "cape
ruby" found in kimberlites; Cape referring to Cape Town, South
Africa. Chromium (Cr-rich) pyropes are
an important indicator mineral in prospecting for kimberlite pipes that
may contain diamonds . The garnets are much more abundant than
diamonds in kimberlite. Sometimes, trails or Cr-rich pyrope in
stream deposits, like
bread crumbs in a forest, can be followed back to their source, a rock
that may contain diamonds.
2) Almandine. Almandine, the most common garnet found,
is at best a crimson red but often is very dark red to
brown/black. It is found in metamorphic rocks
all over the world. Excellent crystals of large size grace
museums, but as a gem it is not the most attractive, often too dark, it
is commonly cut using a hollow cabochon that decreases the darkness by
thinning the gem.
3) Andradite. There are several types of
andradite. The most important is the near emerald-green
demantoid. Demantoid
is the most valuable species of garnet, because it has
properties similar to diamond. It has a dispersion similar to
diamond, which means it splits light into many colors, creating
fire. Demantoid often contains inclusions of fibrous
asbestos. These fibers often resemble a horse's tail and are
referred to as horsetail inclusions. The inclusions, if not too
abundant, are desirable and are looked for by collectors. The
demantoid garnet is based on rich green color and grades into the less
valuable yellow-green topazolite variety. Andradite can also be
found in less desirable shades such a black and colorless varieties
rarely used in jewelry.
Andradite garnets of various colors
4) Grossular. Grossular is quite variable in
color and can be very pleasing in transparent crystals and gems.
Colors range from green to brownish orange, and pink . Colorless
material is also known. Two very important varieties are
Tsavorite and Hessonite. Tsavorite is a bright well-saturated
green colored by chromium and/or vanadium. Hessonite is
sometimes called the cinnamon stone; it is colored red-brown to
brownish orange. Though somewhat less valuable than tsavorite,
both these types are important in jewelry.
5) Spessartine. Spessartine is most important as
gem in its orange color. It varies from orange yellow to reddish
orange.
6) Uvarovite.
Uvarovite typically occurs as very fine crystals. It is
invariably a rich green, often emerald green in color. Chromium
is responsible for this. Uvarovite is so fine grained, that gems
are never cut from it, but the attractive green crystals occur in
clusters or mats of crystals in rock,
and these cluster can be cut to form pieces used in jewelry. The
folksy effect of the many small crystals can be attractive in pendants,
brooches, etc.
Orange and Wine-red
Garnets
Demantoid Garnet
Identifying properties of
garnet
Garnet crystals are often well
formed and usually occur as ball-like dodecahedrons or related
forms.
Because all garnets are isotropic,
they are black when placed between two crossed polarizers in a
polariscope. They
show no birefringent colors. Though spinel is also isotropic, it
usually forms octahedral crystals and may show a little color between
crossed polarizers because of anomolous birefringence.
Garnets in general are not confused with many other stones, although
tsavorites might be confused with other green stones such as emerald or
tourmaline. Polarized light would help here. In general,
consider ruby, tourmaline, and fire opal, which can have similar
colors. If further test are needed, using a spectroscope or
testing for specific gravity (density) may be helpful.
Garnet-Topped Doublets
Garnet-topped doublets were used in
the past to add a durable top to a piece of colored glass. The
doublet's crown was topped with garnet. The garnet might be fused
to the glass pavilion at a position other than the girdle to avoid
detection. The top of the garnet was harder than glass and
could
protect the doublet from scratching. Since the garnet top is
thin, the color of the glass provides most of the stone's
color. Hence garnet topped doublets may be colorless
if the top is thin.
GGG and YAG
These synthetic "garnets" have the
same structural formula as some natural garnets, but have not been
found in nature. Perhaps GGG or Yag could confused with
natural garnets, but they are also mentioned here because they were
important diamond simulants and are still used today by private/amateur
cutters even though they have largely been replaced by cubic zirconium
as a diamond simulant.
GGG stands for gadolinium gallium
garnet and YAG stands for yttrium aluminum garnet. These two
substances have the same formula as natural garnets and the same
crystal shape, but are rare earth element garnets that never occur in
nature. They are man made, using either Czocharalski method of
crystal pulling or the flux fusion method (discussed under synthetic
stones). They have very high dispersion, similar to diamond, and
are very dense because of their rare earth composition. The only
natural garnet they might be confused with is Andradite.
Very large boules or crystals can
be grown, so large stones can be produced. Though these stones
are
not commonly used in jewelry because of supplementation by cubic
zirconium, they are still used in laser applications, etc., and it is
possible that you might find them used occasionally as gemstones.
YAG lasers are still commonly used so the material is out there.
Colorless stones with diamondlike or garnetlike properties, but too
dense to be diamonds could easily be GGG or YAG.