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Western Mineral Resources Garnet—An Essential Industrial Mineral and January’s Birthstone Photograph of a mineral arnet is one of the most specimen containing large G common minerals in the crystals of the garnet world. Occurring in almost any mineral spessartine (red), showing the distinctive color, it is most widely known “euhedral isometric” for its beauty as a gem stone. crystal form of garnet. Used Because of its hardness and since ancient times for other properties, garnet is also an jewelry, the first industrial use of garnet was prob- essential industrial mineral used ably in coated sandpaper in abrasive products, non-slip manufactured in the United surfaces, and filtration. To help States by Henry Hudson Barton in 1878. The United manage our Nation’s resources of States currently consumes such essential minerals, the U.S. about 16 percent of the Geological Survey (USGS) pro- global production of indus- vides crucial data and scientific trial garnet. (Copyrighted photo by Stan Celestian/ information to industry, policy- courtesy of Glendale Com- makers, and the public. munity College). Garnet as a Gem Stone in 1878. Garnet is an important industrial ing aluminum and other soft metals for mineral because it is relatively hard, rating 6 use in aircraft and ships; deburring welds Garnet is familiar to many people as to about 8 on the “Mohs scale of hardness,” and grinding and polishing optical lenses; January’s birthstone and as the New York where diamond is the hardest at 10. Conse- producing high-quality, scratch-free semi- State gem stone. It has been used as a gem quently, garnet is an excellent abrasive for conductor materials; finishing hard rubber, stone since prehistoric times. Most recogniz- applications such as sand- or air-blasting, plastic, and wood products; and making non- able in a deep shade of red, garnet occurs in water-jet cutting, and sandpaper. Garnet is skid paints and coatings. The U.S. petroleum the greatest variety of color of any mineral, now used for some blast-cleaning applications industry is one of the leading garnet-con- except for blue. Some types of garnet even that previously used common sand. Common suming industries, using garnet for cleaning change color when exposed to artificial light. sand contains crystalline silica (SiO2) that in drill pipes and well casings. Garnet is also Unlike many other gem stones, garnet is the form of dust produced during blasting can chemically inert and recyclable, making it an almost never treated using heat, chemicals, cause lung damage if inhaled. ideal filtration medium. or other methods to enhance appearance There are many additional industrial Garnet can also be used in mining and value. The value of gem-stone garnets applications of garnet, including condition- exploration as an indicator to help locate dia- involves a complex combination of factors, mond deposits. Diamonds only form at great including color, clarity, scarcity, durability, pressure and temperature and are associated type of cut, and fashion. Larger garnets are with kimberlite, an intrusive igneous rock popular both as gems and as collectable min- that comes from deep within the Earth. As eral specimens. Star garnets, found in Idaho, the molten rock (magma) that cools to form India, and Madagascar, contain inclusions of a kimberlite rises toward the Earth’s surface, the fibrous mineral rutile that reflect light in it incorporates fragments of rocks and miner- two, four, or six rays when the stones are cut als, including garnets with distinct color and and polished. Star garnets with six rays are composition that form under the same con- the rarest, and for about every 500 four-ray ditions as diamonds. If these distinct types garnets only 1 six-ray garnet is found. of garnet are found in sediments, they may indicate the presence of nearby diamond- bearing kimberlites. Garnet as an Industrial Mineral These cut and polished gem stones are a rare, green variety of andradite garnet, also known as How do Garnets Form? The first industrial use of garnet appears to “demantoid” garnet. Cut garnets are prized for their have been in coated sandpaper manufactured beauty and are also commercially valuable for use Garnet crystals form in rocks of appro- in the United States by Henry Hudson Barton in jewelry. (Image courtesy of Peter Grumitt.) priate chemical composition that have been U.S. Department of the Interior USGS Fact Sheet 2006–3149 U.S. Geological Survey 2006 U.S. Industrial Consumption of The Industrial Uses of Garnet Garnet The United States presently consumes about 16 percent of the global production of industrial garnet. In 2005, domestic produc- tion of crude industrial garnet was 40,100 metric tons (t), valued at $3.84 million. In the 10% other same year, the Nation imported 41,800 t of 10% industrial garnet, valued at $5.91 million, pri- Abrasive 35% Powders marily coming from Australia, Canada, India, Abrasive Blasting and China. 15% Water Filtration Meeting the challenge of supplying America’s needs for essential minerals such 30% as garnet requires accurate and unbiased sci- Waterjet Cutting entific data. The ongoing work of scientists with the USGS Mineral Resources Program provides the information crucial to the cre- ation of sound policies that will ensure future supplies of mineral resources. This photo shows garnets of various grain sizes. Garnets are hard, chemically inert, and recyclable, making them ideal for industrial uses (see pie diagram). Garnets similar to James G. Evans, Phillip R. Moyle, David G. those shown here are used in water-jet cutting, air-blasting, and filtration applications. Frank, and Donald W. Olson Edited by James W. Hendley II Graphic design by Stephen L. Scott modified by heat and pressure deep within Almandine is the hardest and, together References the Earth’s crust. These are “metamorphic” with andradite, is the garnet mineral most rocks, such as gneiss, amphibolite, and commonly used in industrial applications. Evans, J.G., 2006, U.S. industrial garnet, in Bliss, schist. Less frequently, garnets form in Tsavorite, a rare green grossularite garnet J.D., Moyle, P.R., Long K.R., eds., 2002, Con- “calcareous” rocks that have been heated of exceptional beauty is among the most tributions to industrial-minerals research: by nearby intrusions of magma. expensive garnets by weight. At the other U.S. Geological Survey Bulletin 2209-L Garnets can be recovered using either extreme are reddish pyrope and abundant [http://pubs.usgs.gov/bul/b2209-l]. surface or underground mining methods. almandine garnets that are both common Olson, D.W., 2005, Garnet, industrial: U.S. Most garnets are found in and recovered and relatively inexpensive. Geological Survey Minerals Yearbook 2005, from alluvium (unconsolidated or poorly p. 29.1-29.3 [http://minerals.usgs.gov/ consolidated combinations of clay, silt, Where are Garnet Deposits Found minerals/pubs/commodity/garnet/index. sand, and gravel) derived from metamor- html#myb]. phic rocks that have undergone weathering in the United States? Hearn, B.C., and McGee, E.S., 1983, Garnets in and erosion. Subsequent stream action Garnet deposits have been located in Montana diatremes; a key to prospecting moves and deposits the garnets, resulting 21 different states, including Alaska, Ari- for kimberlites: U.S. Geological Survey Bul- in local concentrations of garnet due to zona, California, Colorado, Connecticut, letin 1604, 42 p. its high specific gravity (its mass relative Idaho, Montana, Nevada, New Hampshire, U.S. Bureau of Mines, 1995, An overview of pro- to an equal volume of water). During the New York, North Carolina, Pennsylvania, duction of specific U.S. gemstones [garnet]: recovery process the specific gravity and Utah, and Virginia. However, the only U.S. U.S. Bureau of Mines Special Publication magnetic properties of garnet are used to garnet mines currently active are in north- 14-95 [http://minerals.er.usgs.gov/miner- separate it from other material. ern Idaho, southeastern Montana, and east- als/pubs/commodity/gemstones/sp14-95/ ern New York. In Idaho, garnet deposits are garnet.html]. found adjacent to metamorphic rocks that Are all Garnets the Same? make up the western Clearwater Range. In Although all garnets possess similar Montana, garnet deposits are found in the physical properties, including crystal struc- Ruby Range and southern Tobacco Root ture, hardness, and specific gravity, there Mountains. In both Idaho and Montana, For more information contact: are actually several different types of the garnets are mined from alluvial deposits James G. Evans U.S. Geological Survey mineral. The six most common members that formed from weathering of amphibo- 904 W. Riverside Ave., Room 202 of the garnet family are almandine (rich lite, mica schist, gneiss, or granite. In Spokane, WA 99201 northeastern New York, garnet is mostly Tel: (509) 368–3115 in iron and aluminum), grossular (rich in http://minerals.usgs.gov calcium), pyrope and spessartine (rich in extracted from unweathered gneiss bedrock magnesium or manganese), uvarovite (rich of the Adirondack Mountains using surface This Fact Sheet and any updates to it are available online at http://pubs.usgs.gov/fs/2006/3149/ in chromium), and andradite (rich in iron). mining methods. Printed on recycled paper.
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