Stone of the Week - Garnet

Garnets are actually a group of minerals, of which there are 15 different mineral species. Each mineral has a similar chemical make up -> X3 Z2 (SiO4)3 where X and Z can be different elements. It is these different combinations that make up the variation of minerals, although Calcium (Ca), Magnesium (Mg), Aluminum (Al), and Iron (Fe) are the most frequent. Out of the different minerals, there are several common garnets: pyrope (dark red), almandine (red to violet red), spessartite (yellow, rose, orange, reddish brown), grossular (white, yellow, yellow-green, brownish-red, orange, black), and andradite (colorless, yellow-green, brown, black). Within each of these are several varieties as well - hessonite and tsavorite are both types of grossular garnets for example.

This display at the National Museum of Natural History in the US shows some of the variety of color just within the grossular species.


Some of the most valuable garnets are also the rarest. Blue garnets are a type of pyrope garnet, that was discovered in the late 1990s in Bekily, Madagascar, and has since been found in the US, Russia and Turkey. It is one of the few garnet species that changes color from blue-green in the daylight to purple in incandescent light. Another sought after garnet is Tsavorite, which belongs to the grossular garnets, and is a brilliant green color. Tsavorite garnets were first discovered in a deposit in Tanzania during the 1960s, which extended into Kenya. The only other known location for these rare garnets is in Madagascar. Uvarovite is another bright green garnet that is one of the less common species, and is found in Russian and Finland.


Uvarovite is a spectacular green garnet, and is often sought after for jewelry.

Garnets are common world wide, particularly in metamorphic rocks such as marble and schist; pyrope garnets are usually associated with igneous rocks. They are abundant in the US, Brazil, South Africa, England, and Australia. Gemstone quality crystals are used in jewelry work, the most common being the red garnets. It is also the birthstone for January. Garnets have also been used as an abrasive for sand blasting, in cutting, and as part of some water filtration units.

Garnets can be found in nice crystal forms, such as the grossular garnets on the left. However the deep reds are a popular color found in jewelry.

If there's a stone you would like to know more about - drop me a line! Have a mystery stone? Feel free to post a comment about it, including a link to a picture, and it could be featured on this blog!



Garnet Facts:
Chemical composition: X3 Z2 (SiO4)3
Crystal System: Isometric/Cubic
Color: Varied – red, green, yellow, brown, black, blue, orange, clear
Habit: Well formed crystals are common, as well as massive and granular.
Fracture: Conchoidal
Cleavage: Indistinct
Luster: Vitreous, Resinous
Hardness: 6.5-7.5
Specific Gravity: 3.4-4.6
Streak: Colorless/White
Occurrence: Worldwide


References
A Guide to Rocks and Fossils by B. Busbey III, R. R. Coenraads, P. Willis, and D. Roots. Published 2002 by Fog City Press. ISBN: 1877019518

Rocks, Minerals, & Fossils of the World by C. Pellant and R. Phillips. Published 1990 by Little, Brown and Co. ISBN: 0316697966

Wikipedia – Garnet. http://en.wikipedia.org/wiki/Garnet