Garnet is a naturally occurring gemstone. Its name comes from Latin granatus meaning seed,
because it often resembles small round seeds when found in its matrix rock.
Rather than a single gemstone, garnet is a family of related minerals, some of which occur
as gemstones. Each has a common crystal structure, and a similar chemical composition.
The popular understanding of garnet is as an inexpensive dark red stone. Because it is
relatively common and inexpensive, it is often thought of as "only Garnet", and as being
inferior. This bias extends to other rare and attractive forms of Garnet. Spessartite is one
of these rare and truly beautiful varieties and is pictured above.
Coloration
Garnet occurs naturally in a large range of colors including: red, orange, brown, green,
yellow, and brown. Its variability of color reflects the variations in its composition.
Chemical Composition and Name
Almandite is an iron aluminium silicate, 3FeO - Al2O3 - 3Si2
Pyrope is a magnesium aluminium silicate
Spessartite is a manganese aluminium silicate
Andradite is a calcium iron silicate
Grossular is a calcium aluminium silicate
Uvarovite is a calcium chromium silicate
Chemical Composition
A general chemical formula would be: D3T3(SiO4)3; where D = a divalent metal - calcium,
magnesium, ferrous iron, or manganese, and T = a trivalent metal - aluminium, ferric iron,
or chromium.
Another way of showing the same composition more clearly would be: 3DO - T2O3 - 3SiO2
Garnet Groups
There are two main theoretical groups or "families" of garnet: pyrope, almandite, spessartite,
which are all (metal) aluminium silicates, and uvarovite, grossularite, andradite, which are
all calcium (metal) silicates. In practice, there are probably very few garnets with the
precise pure chemical composition shown for their type, almost all garnets are of mixed types,
where one type is partially replaced by another type.
Very few, if any, pure "types" of garnet occur, most specimens approximate to a particular
type. Brown and purpleish "Indian" garnets will usually be towards the pyrope end of the
pyrope-almandite axis. Reddish "African" garnets will usually be towards the almandite end of
the pyrope-almandite axis. Rhodolite garnets are usually about midway along the
pyrope-almandite axis.
Demantoid Garnet
Demantoid garnet is a rare and beautiful bright grass green sub-variety of andradite garnet.
It appears to have first been discovered around 1892 in the Bobrovka area of Russia.
The Bobrovka is a small tributary of the River Tschussowaja in the Sissersk region on the
western side of the Ural Mountains. It was at first thought to be emerald, which is found
nearby, and has been erroneously called "Uralian emerald".
The name demantoid means diamond-like, because it has a very high adamantine lustre, and a
color dispersion higher than diamond. The only disadvantageous property of demantoid is its
low hardness figure at about 6.5 Moh. It is the softest of the garnets, and is more suitable
for use in brooches, pendants, or ear-rings, rather than rings, because of this.
The brilliant color of demantoid garnet is due to partial replacement of the silicate by
chromic oxide. A diagnostic characteristic of demantoid is the inclusion of radiating fibres
of byssolite (asbestos) fibres in a pattern described as a horse-tail. There is no other green
stone which shows this feature.
In late Victorian times, and early in the twentieth century, demantoid became a very sought
after stone. It commanded high prices because it has never been available in large quantity.
In recent decades, it has been unobtainable as newly mined stones, and has only been available
from antique jewelry. Recently, small finds have again been made in Russia, and a small
quantity of fine quality stones have recently come onto the market. Gemstone lovers wishing
to acquire a piece of demantoid garnet should take this opportunity to do so. If the current
seams of demantoid run out, there may be another century without new stocks of demantoid
becoming available.
Tsavorite Garnet
Tsavorite is a bright green variety of grossular garnet, its color being induced by the
presence of chromium.
Topazolite
A name used for a variety of garnet with a topaz-yellow or an olive green color.
Magnetism
Some types of garnet, particularly the reddish "African" garnets, which are usually found in
close association with diamonds, showing distinct magnetism.
Technical Characteristics
Hardness
Ranges from 6.5 to 7.5 and is variable, 7.0 - 7.5 within a single specimen. "Low" types 6.5
Almandine 7.5
Andradite 7.5
Demantoid 6.5
Grossular 7.0
Pyrope 7.25
Rhodolite 7.0 to 7.5
Spessartite 7.25
Uvarovite 7.5
Refractive Index
Each variety of garnet has its own range of RI and it varies considerably. The "high" type
being 1.92 to 1.98; the bi-refringence 0.059 uniaxial and positive.
Almandine 1.78 to 1.83
Andradite 1.78 to 1.83
Demantoid 1.888 to 1.889
Grossular 1.72
Hessonite 1.742 to 1.748
Melanite 1.89
Pyrope 1.705 to 1.73
Spessartite 1.79 to 1.81
Uvarovite 1.87
Dispersion
Almandine 0.027
Andradite
Demantoid 0.057
Grossular
Hessonite 0.027
Melanite
Pyrope 0.022
Spessartite 0.027
Uvarovite
Density (Specific Gravity)
Almandine 3.95 to 4.25
Andradite
Demantoid 3.82 to 3.85
Grossular 3.36 to 3.55
Hessonite 3.65
Melanite 3.90
Pyrope 3.51 to 3.65
Spessartite 4.12 to 4.20
Uvarovite 3.77
