Garnet, the January birthstone, derived its name from the Latin word granatus, meaning like a
grain, which refers to the mode of occurrence wherein crystals resemble grains or seeds embedded
in the matrix. Garnet is a family of minerals having similar physical and crystalline properties.
They all have the same general chemical formula,
where A can be calcium,
magnesium, ferrous iron, or manganese, and B can be aluminum, ferric iron, or chromium, or in
rare instances, titanium.
The formulas and names of common garnet species are:
Figure 4. Star garnet from Idaho.
(Photo is courtesy of the author.)
Garnet displays the greatest variety of color of any mineral, occurring in every color except blue. For example, grossularite can be colorless, white, gray, yellow, yellowish green, various shades of green, brown, pink, reddish, or black. Andradite garnet can be yellow-green, green, greenish brown, orangy yellow, brown, grayish black or black. Pyrope is commonly purplish red, purplish red, orangy red, crimson, or dark red; and almandite is deep red, brownish red, brownish black or violet-red. Spessartite garnet can be red, reddish orange, orange, yellow-brown, reddish brown, or blackish brown. A few garnets exhibit a color-change phenomenon. They are one color when viewed in natural light and another color when viewed in incandescent light.
The use of garnets as a gem or gemstone can be traced to prehistoric times. However, the first industrial use of garnet appears to have been as coated sandpaper manufactured in the United States by Henry Hudson Barton (founder of Barton Mines Corp.) in 1878. Its use has grown from that single sample of garnet coated sandpaper, to world industrial uses of more than 110,000 tons per year. In 1994, United States production of industrial garnet was valued at about $14 million, while gem garnet production was valued at only about $233,000.
Many deposits within the United States produce fine gem-quality garnets and three deposits are mined for industrial garnet. A State-by-State review is presented below.
Alaska.--Garnets from deposits along the Stikine River, often called "Wrangell garnet" after the nearest town which is located on Wrangell Island, are famous as mineral specimens because of their near perfect crystalline form. The garnets have limited use as gemstones because of their deep red color, however some cabochons are cut from them.
Arizona.--Arizona is one of five States that has commercial production of gem garnets. Arizona's gem garnet is red pyrope from two locations in the extreme northern portion of Apache County on the Navajo Indian Reservation. One location is at Garnet Ridge which is about 8 km west of the town of Mexican Water, and the other location is in Buell Park on the Arizona and New Mexico border, about 16 km north of Fort Defiance. Faceted stones cut from materials from these locations average 1/2 to 1-1/2 carats in size, but stones as large as 5 carats are known from these locations. Additionally, fine-quality andradite specimens, some suitable for cutting, are available from an area near Stanley in Graham Co.
California.--Gem- and specimen-quality white to pale green grossularite garnet occurs on Indian Creek in Siskiyou County and along Traverse Creek near Georgetown in Eldorado County. Other locations for these types of grossularites are the south side of Watts Valley in Fresno County, near Selma in Tulare County, near Big Bar in Butte County and near El Toro in Orange County. Some of the finest quality spessartite garnet known come from pegmatites in San Diego County. Spessartites have been found on Gem Hill near Mesa Grande and in mines in the Rincon and Pala Districts. The most productive area with the finest quality garnets is on the western side of Hatfield Creek Valley near Romona. Near Indian Head Hill in San Diego County is a deposit of fine-quality hessonite garnet, and another deposit is near Dos Cabezas.
Colorado.--Faceting-grade spessartite garnets can be found in the gas cavities in the rhyolite flow on Ruby Mountain near Nathrop, Chaffee County. Large specimen-grade garnets are available at the abandoned Salida Copper Mine.
Connecticut.--Garnet in mica schist near Roxbury and Roxbury Falls in Litchfield County was once mined, although not recently, for use in abrasive applications and for mineral specimens. The andradite garnet was found as well-formed dodecahedral crystals that separate easily from the host schist.
Idaho.--Alluvial deposits of almandite garnet were discovered in the early 1880's near Fernwood in Benewah County, but commercial gem and industrial mining did not start until the early 1940's. The deposits are on Emerald, Carpenter, and Meadow Creeks about 6.4 km from a mica-garnet schist formation. The garnet-bearing gravels are about 1 m to about 1.2 m thick and contain 8% to 15% garnet. These deposits are the basis of the largest industrial garnet mine in the nation and also produce gem garnet. Additionally, star garnets are produced from the placers of Purdue Creek in Latah County.
Idaho's deposits also are the only ones in the world, besides India, that produces significant amounts of star garnets. These almandite garnets are translucent, purplish-red stones that show four- or six-ray stars when cabochon cut, or are transparent, deep red stones that can be faceted. The asterism is the result of silky rutile inclusions.
Gem-quality garnets also are produced commercially from an area of the Little North Fork and North Fork of the Clearwater River in Clearwater County. They range from purplish rose-red to a highly prized "special pink." Gem-quality garnets occur at a number of other locations in Idaho and are periodically mined by hobbyists or professional collectors for the gemstone market.
Montana. -- Several alluvial deposits of almandite-pyrope garnet are located on the drainages of the Ruby River in Madison County. One such deposit, the Alder Gulch deposit, is in the alluvial fan formed where Alder Gulch joins the Ruby River Valley. The deposit contains about 40 million tons of old placer gold tailings that grade 4.5% almandite-pyrope garnet. The alluvium consists of high-energy, fluvial, well-rounded material, approximately 50% of which is +9 mm in size. The source of garnet along the Ruby River is highly metamorphosed Archean rocks in the Tobacco Root and Greenhom Mountain Ranges to the east. The garnets are present mostly as broken fragments of crystals which were originally as much as several centimeters in diameter. Some of the garnets from this industrial garnet deposit are of gem quality.
Bright red and orange red, to reddish-pink pyrope-almandite garnets are found associated with sapphires in the gravels of the Missouri River near Helena in Lewis and Clark County.
Nevada.--An alluvial deposit of almandite garnet is found along Hampton Creek Canyon in White Pine County about 3 km from the mouth of the canyon. No production history is available for the deposit. The source of the garnet is quartz-garnet-mica-staurolite schist that forms a portion of the walls of the canyon.
Spessartine garnets can be found at several locations in White Pine County. Most of the dark brown crystals are of interest only as specimen, but a few will cut very small clear stones.
New Hampshire.--A garnet deposit is located near North Wilmont in Merrimack County, where small alrnandite crystals are found in a biotite, quartz, and albite feldspar matrix. The crystals range up to about 19 mm in diameter but average only 6 to 10 mm in diameter.
New York.--Deposits of industrial garnet are found at two locations in New York. In the Gore Mountain area, industrial garnet is mined as the primary product; and near the town of Willsboro, byproduct of it is recovered as wollastonite mining.
The deposit near Gore Mountain is an almandite-bearing diorite of uncertain, igneous or metamorphic origin. The garnet is present as imperfectly developed crystals surrounded by a rim of coarsely crystalline hornblende. The crystals range from about 1 millimeter to almost 1 meter in diameter but average about 100 millimeters in diameter. The garnet has a pronounced laminated structure, which enables it to naturally break into thin plates from about 2 to 6 millimeters in thickness. Garnet fragments maintain this platy particle shape even as they are crushed smaller and smaller.
These same deposits in Warren County contain good to fine quality facet-grade garnets. The garnet is a solid solution of pyrope-almandite-grossularite that results in a pleasant deep brownish-red material which often has an orange cast. Beautiful small stones can be cut, but larger stones are too dark to be attractive.
North Carolina.--Large deposits of almandite and rhodolite garnet of gem and abrasive quality are known in Clay, Jackson, Macon, Madison, and Burke Counties. Abrasive-grade garnet was produced from some of these deposits from 1900 to about 1926, but no production has been recorded recently. The deposit of almandite garnet in Clay County is in a hornblende gneiss at Penland Bald on Buck Creek.
Fine red colored, gem-quality pyrope garnets have been found in the wastes from placer gold operations in Burke, McDowell, and Alexander Counties. Rose-pink rhodolite garnets are recovered from gravels in Cowee Creek near Franklin, and Mason's Branch near Iotla, both in Macon County. Rhodolite can be found in situ on Mason Mountain.
Pennsylvania.--Almandite garnet crystals are found in a quartzose mica schist about 1.6 km west of Chelsea in Delaware County. Near the surface the schist is badly weathered and the garnets, which comprise as much as 75% of the rock locally but average much less, are easily recovered. Abrasive-grade garnet was produced from this deposit prior to 1900, but no production has been recorded since the turn of the century.
Small trapezohedral crystals of almandite garnet are scattered through-out a badly decomposed gneiss near Chester Heights, also in Delaware County. An attempt was made to mine this deposit years ago by means of a shaft and underground workings.
Utah.--Large specimen-grade, apple-green grossularite garnets have been found in western Beaver County. Two locations southeast of Mexican Hat, San Juan County produce pyrope garnets similar to the bright red pyropes from Arizona and New Mexico.
Virginia.--A deposit of garnet is located on a steep bluff on the northeast side of the Tyle River about 6.5 km south of Arrington in Nelson County. The small, dull-red garnets are found in a sericitic schist. Attempts were made to mine the deposit in the past, but there is no record of commercial production.
Two mines in Amelia County account for the majority of the production of gem-quality garnet, they are the Morefield and Rutherford. The spessartine from these mines, primarily the Rutherford, are etched-crystal masses and fragments, not individual perfect crystals. The pieces range from pea size to as large as a grapefruit. In 1991, a single piece, dubbed the Rutherford Lady, was found that weighed more than 2,800 carats. Color varies from a fantastic light pure orange, almost yellow to shades of red-orange, red, and brownish-red, but the orange overtone always is present.
U.S. production of gem-grade garnet will continue to increase and additional deposits will be
brought into production in the coming years.
U.S. Department of the Interior
U.S. Geological Survey
988 National Center
Reston, VA 20192 USA
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Last modification: 11-Jan-2013@16:56