Garnet
There are six common varieties of garnet.
Pyrope
The blood-red colour of pyrope is due to its iron and chromium content. It rarely has inclusions, but when present they are rounded crystals or have irregular outlines. As with all garnets, pyrope has no cleavage, and fracture is subconchoidal to uneven.
Occurrence
Pyrope is found in volcanic rock and alluvial deposits, and may, along with certain other minerals, indicate the presents of diamond-bearing rocks. Localities include Arizona (USA), South Africa, Argentina, Australia, Brazil, Burma, Scotland, Switzerland, and Tanzania.
Remark
Pyrope comes from the Greek pyropos, meaning fiery. Swiss and South African pyropes are lighter red than stones from Bohemia, where pyrope jewellery has been made for over 500 years.
| Crystal structure | Cubic |
| Composition | Magnesium aluminium silicate |
| Hardness | 7 |
| SG | 3.80 |
| Rl | 1.72-1.76 |
| DR | None |
| Lustre | Vitreous |
Spessartine
Gem-quality spessartine is uncommon. It is bright orange when pure, but an increase in the iron content makes the stone darker orange to red. Inclusions are lace- or feather-like.
Occurrence
Spessartine occurs in granitic pegmatities and alluvial deposits. It is found in Sri Lanka, Madagascar, Brazil, Sweden, Australia, Burma, and the USA; also Germany and Italy, but crystals there are too small to facet.
Remark
Spessartine is named after the spessart district of Bavaria, Germany. It can be confused with hessonite garnet or yellow topaz, but on close examination of inclusions it is distinguishable.
| Crystal structure | Cubic |
| Composition | Magnesium aluminium silicate |
| Hardness | 7 |
| SG | 4.16 |
| Rl | 1.79-1.81 |
| DR | None |
| Lustre | Vitreous |
Almandine
Almandine is generally darker red than pyrope and may appear black, although pinkish red specimens are found. It is usually opaque or sub translucent, but the rare transparent stones have high lustre. Although dense, almandine is brittle and facet edges chip. Many stones show characteristics inclusions, and four-rayed stars may be seen when the stones are cut en cabochon. The darker almandines are frequently cut as cabochons or used as abrasives in garnet paper. The underside of dark almandine is often hollowed out to let more light filter through the stone.
Occurrence
Almandine is found in metamorphic rocks, such as garnet mica schist, and less frequently in granitic pegmatites. It has a worldwide occurrence.
Remark
Slices of garnet have been used in windows in churches and temples, and legend has it that Noah suspended garnet in the ark in order to disperse light. Garnet was once said to cure melancholy and to warm the heart.
| Crystal structure | Cubic |
| Composition | Iron aluminium silicate |
| Hardness | 7.5 |
| SG | 3.77 |
| Rl | 1.86-1.87 |
| DR | None |
| Lustre | Vitreous |
Hessonite
Grossular garnets occur in a very wide range of colours, from colourless right through to black, but derive their name from the first specimen ever found, a distinctive gooseberry-green colour. The orange-brown colour of hessonite grossular garnet is due to manganese and iron inclusions.
Occurrence
The best hessonite garnets are found in Sri Lanka in metamorphic rocks, or gem gravels and sands. In Madagascar hessonite is often referred to as cinnamon stone. Other localities include Brazil, Canada, and Siberia (Russia), as well as Maine, California and New Hampshire in the USA.
Remark
Both the Ancient Greek and Romans made cameos, intaglios, and cabochons from hessonite, and faceted stones for jewellery.
| Crystal structure | Cubic |
| Composition | Calcium aluminium silicate |
| Hardness | 7.25 |
| SG | 3.65 |
| Rl | 1.73-1.75 |
| DR | None |
| Lustre | Vitreous or resinous |
Green Grossular
There are two varieties of green grossular: one is found as transparent crystals, the other is massive. Massive green grossular from South Africa is called Transvaal jade, after it’s main locality and because it resembles jade. It may contain black specks of the mineral magnetite. Since the 1960s a transparent, green grossular garnet, named tsavorites, has been mined in Kenya. Massive green grossular is used as a decorative stone; tsavorites is faceted as a gem.
Occurrence
Found in Canada, Sri Lanka, Pakistan, the former USSR, Tanzania, South Africa, and the USA. Kenya is the main source for tsavorites.
Remark
The name “grossular” is derived from the botanical name of the gooseberry, R. grossularia. Massive grossular garnet of a gooseberry-green colour was first discovered in the former USSR. Since then it has also been found in Hungry and I taly.
| Crystal structure | Cubic |
| Composition | Calcium aluminium silicate |
| Hardness | 7 |
| SG | 3.49 |
| Rl | 1.69-1.73 |
| DR | None |
| Lustre | Vitreous |
Andradite Garnet
Garnets containing titanium and manganese are grouped as andradite garnet. The most valuable is demantoid, whose emerald-green colour is due to the presence of chromium. It has a higher dispersion than diamond and can be recognized by the characteristics “horsetails”, which are fine, hair-like inclusions of asbestos. Topazolite, the yellow variety of andradite garnet, varies from pale to dark yellow. Only small crystals are found. Melanite is a generally black form, but can also be dark red.
Occurrence
The best demantoid is found in the Urals in Russia, and is associated with gold-bearing sands and metamorphic rocks. Other localities include northern Italy, Zaire, and Kenya. Topazolite crystals are found in the Swiss and Italian Alps in metamorphic rocks. Melanite is found in metamorphic rocks and volcanic lavas; fine crystals are found on the island of Elba (Italy), and in France and Germany.
| Crystal structure | Cubic |
| Composition | Calcium iron silicate |
| Hardness | 6.5 |
| SG | 3.85 |
| Rl | 1.85-1.89 |
| DR | None |
| Lustre | Vitreous to admantine |
