Image courtesy of Amethist Galleries, Inc. 2004

Quick garnet info:

Mohs' hardness: 6.5-8.5

Index of refraction: 1.73-1.889

Fracture: conchiodal

Streak: none

Cleavage: none

Specific gravity: 3.5-4.3

Crystal form: dodecahedral

Related Links

http://www.mineralminers.com/html/uvamins.stm

http://www.gggems.com/garnet_crystal.htm

http://www.johnbetts-fineminerals.com/jhbnyc/garnet.htm

http://www.faceters.com/gemstones/garnet.shtml

http://www.echinastone.com/

Garnets

Introduction

Garnets are a very common and well-known mineral formed through metamorphic processes. They are, perhaps, best known for their status as the birthstone for the month of January. Like most minerals, garnets are believed by some to posess magical powers. These include protection, healing, physical energy, and theft management, as well as promoting love, controling sexual energies, calming, and unlocking memories of past lives. Really.

The United States is the worlds third largest producer of garnets, behind Austrailia and India, producing 38,600 metric tonnes of garnets in 2003.

Geologic Processes

Garnets are a commonly identified index mineral. When higher grade metamorhpic processes occur, garnets are often formed. They are therefore commonly present in schists and gneisses. Radiometric dating (using U-Pb, Rb-Sr, and Sm-Nd) of the garnets can be used to determine the age of the rocks were formed in. Once the age of the rock is known, the cooling speed can be determined by anylizing the Sm-Nd decay.

Garnets are the historians of the minerals. Most garnets are formed by standard metamorphic processes. Others, however, are formed by being rolled between two moving surfaces. This rolling can cause them to pick up and preserve lower grade metamorphic rocks that would normally be destroyed by higher processes sugh as the ones that form garnets. This can give geologists valuable information about how and where the rocks were formed.

Different types of garnets

Pyrope garnets are a favorite for jewelers. They occur in deep to light blood red or black color. Pyrope garnets are found in Bohemia, South Africa and Arizona.

Image courtesy of Hershel Friedman 2004

Almandine garnet is the most commonly used garnet for jewelry, due to its abundance, color, and hardness. It is the hardest of the garnets, reaching hardnesses of 8.5. These garnets are mined in Brazil, India, Sri Lanka, Aurtralia, and the USA.

Image courtesy of Hershel Friedman 2004

Spessartine garnet may be brown, orange, or pink. Spessartine garnets are found in Bavaria, Sri Lanka, and the USA.

Image courtesy of Hershel Friedman 2004

Grossular garnet can be found colorless, white, green, yellow, pink, brown, or orange. The most common yellow and brown stones are found mostly in Sri Lanka and secondarily in Mexico and Oregon.

Image courtesy of Hershel Friedman 2004

Andradite garnet occurs in red, green, yellow, brown or black. Adradite garnets are mined in the Ural mountains and in the USA.

Image courtesy of Amethist Galleries, Inc. 2004

Uvarovite is the rarest of the garnets and the only chromium containing one. It is an emerald green color with a very high index of refcation, but is seldomly used for jewelry because it rarely occurs in large enough crystals to facet. Uvarovite garnets are found in Russia and Finland.

Image courtesy of Amethist Galleries, Inc. 2004

Uses

Garnets are very popular minerals for jewelry. They are comparable in color to many other gems such as amathyst, fire opals, diopside, emeralds, and rubies but are better for many reasons. Garnets have a much higher hardness than fire opals or diopside, are much less expensive than emeralds or rubies (or even dipsodes), and a higher index of refraction than all of them. In fact, garnets have a higher index of refraction than all ofther commonly used colored gems. These factors combine to make garnets ideal for jewelry that is practical, economic, and beautiful.Industrial grade garnets may be used as an abrasive called garnet paper. It is less durable than corundum based sand paper, but it is less expensive. Other uses include abraisive blasting media, waterjet cutting, and abraisive powders. Garnet is a higher quality abraisive than the more commonly used and cheaper abraisive aluminum oxide. Silicon carbide and quartz sand are two close rivals of garnet abraisives.

Literature Cited

Stephen Mershak. Essentials of Geology. W.W. Norton & Company, Inc., 2004.

Ganguly, J., M. Tirone, R.L. Hervig. 1998. Diffusion kinetics of samrium and neodymium in garnet, and a method for determining cooling rates of rocks. Science 281 (5378): 805-807

Liou, J.G., S. Maruyama, W.G. Ernst. 1997. Seeing a mountain in a grain of garnet. Science 276 (5309): 48-49

www.earthbow.com/crystals/garnet/htm

http://mineral.galleries.com/minerals/silicate/garnets.htm

http://www.minerals.net/mineral/silicate/neso/garnet/garnet.htm

http://minerals.usgs.gov/minerals/pubs/mcs/2004/mcs2004.pdf

http://reference.allrefer.com/encyclopedia/G/garnet.html

http://www.stansburyholdings.com/ore_info/garnet_info.htm

http://www.uk-high-street.co.uk/guides/magicalstones.html

Link to other Student Webpages for 2004 Earlham Physical Geology

Copyright © 2004 Earlham College. Revised April 19, 2004 . Send corrections or comments to parkero@earlham.edu