Garnet

The garnet group of minerals usually shows crystals with a habit of rhombic dodecahedrons and also with trapezohedrons. They are nesosilicates with the same general formula, A3B2 (SiO4)3. The chemical elements in the garnet include calcium, magnesium, aluminum, iron2+, iron3+, chromium, manganese, and also titanium. Garnets show no cleavage and a dodecahedral parting. Fracture is a concordat to uneven; some varieties are very tough and are also very valuable for abrasive purposes. Hardness is 6.5 - 8.5, specific gravity is 3.1 - 4.3, luster is vitreous to resinous, and that they can be transparent to opaque.

History and Colour

The name "garnet" comes from the Latin granites and a grain possibly in reference to malum granites (pomegranate) it is 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 out in a variety of colors including purple, red, orange, yellow, green, brown, black, or colorless as well. The lack of a blue garnet was remedied in 1980's following the discovery of color-change blue to red/pink material in the Bekily, Madagascar but these stones are very rare. Color-change garnets are by far the rarest garnets except the gem uvarovite, which do not come in cuttable sizes. In daylight, their color could be shades of green, beige, brown, gray and rarely blue, to reddish and 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 are also more dramatic than the color change of top quality Alexandrite which is frequently disappointing, but still sold for many thousands of dollars (US) per carat. It is expected that blue color-change garnets would match Alexandrite prices and 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).

Uses of garnets

• Pendant in uvarovite, a rare bright-green garnet.

• Pure crystals of garnet are used as gemstones. Garnet sand is a good abrasive

• Pyrope varieties are used as kimberlitic indicator minerals in diamond prospecting.

• Garnets are very abundant in the lower crust and mantle and thus play an important role in geochemical understanding of the Earth.

• The garnet is the official mineral and color of Bates College.