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Garnet occurs naturally in a large range of colours including: red, orange, brown, green, yellow, and brown.
Its variability of colour 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
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
Two Families of Garnet
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.
In the trade, gem dealers hardly ever refer to their stock as "pyrope" or "almandite". Instead, they will often refer to "brown" or Indian garnets. As we have stated, very few, if any, pure "types" of garnet occur, most specimens approximate to a particular type.
Brown and purpish "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 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 colour 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 colour 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 jewellery.
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.
Some types of garnet, particularly the reddish "African" garnets, which are usually found in close association with diamonds, showing distinct magnetism.
7 to 7.5 variable, even within a single specimen. "Low" types 6.5
Rhodolite 7.0 to 7.5
Each variety of garnet has its own range of RI
Almandine 1.78 to 1.83
Demantoid 1.888 to 1.889
Hessonite 1.742 to 1.748
Pyrope 1.705 to 1.73
Spessartite 1.79 to 1.81
The RI of garnet varies considerably, the "high" type being 1.92 to 1.98; the bi-refringence 0.059 uniaxial and positive.
Density (Specific Gravity)
Almandine 3.95 to 4.25
Demantoid 3.82 to 3.85
Grossular 3.36 to 3.55
Pyrope 3.51 to 3.65
Spessartite 4.12 to 4.20
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