Geologic Definitions:

In geology, a mineral is defined as “a homogeneous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice” (Marshak 81).

A crystal is “a single, continuous piece of crystalline material bounded by flat surfaces (crystal faces) that formed naturally as the crystal grew” (Marshak 83).

Polished and cut garnets displayed according to size for commercial purposes (Courtesy of http://www.idahogems.com/gemstone.htm ).

Interesting Facts About Garnets:

- The word garnet is derived from the latin "granatus," meaning "like a grain," referring to the similarities of the crystal structure to the grains or seeds embedded in a matrix. Garnets were given this name because their shape and color were thought to resemble pomegranate seeds.

- Garnet is the January birthstone

-For the past 5,000 years, garnets have been valued as gems.

- The Romans used garnets for carving.

- Garnets were especially popular in the 19th Century Victorian garnet jewelry

- Garnets are said to possess some magical qualities. Some believe garnets to ward off danger while traveling. They are also sometimes thought to ensure that the wearer stay in good health.

Rough garnets. These gemstones have just been dug up in northern Idaho. They must be carefully examined by a jeweler in order to determine their commercial worth (Courtesy of http://www.idahogems.com/gemstone.htm ).

Physical Properties of Garnets:

Hardness = 6.5-8.5

Specific gravity = 3.54 - 4.33

Cleavage = None

Fracture = Subconchiodal

Luster = Resinous

Transparency = Transparent to Translucent

Color = Every shade except blue

Crystal form = Dodecahedral

STAR GARNETS: A Mystery?

The highly publicized and rarely found six ray star garnet. The ratio of 4 ray garnets to 6 ray garnets is believed to be about 500 to 1 or more (Courtesy of http://www.idahogems.com/gemstone.htm ).

The Star Garnet is Idaho's official state gem. It is rarity in the usually abundant mineral, for they are only found in northern Idaho and India. Star Garnets come in the dark purple almandine form of garnet. The stars are marked by rays that pass through and cross the garnet when it is held in the light. The Star Garnet comes in two varieties: four-rayed and six-rayed. The six-rayed garnets are extremely rare and valuable. The stars are said to be the result of tiny rods of rutile located within the garnet. These rods are structured around the garnet’s crystal planes.

That said, this phenomenon does not concur with geologists' knowledge of the mineral's structure. While the four-rayed garnets are possible, the six-ray are not because the facial planes of a dodecahedron can not accommodate six-rays.

To visualize this dilemma, go back to the side-by-side photographs of the dodecahedron garnet (above). You could draw upon the planes of the garnet to create two lines that intersect towards the center of the garnet. However, you could not draw three lines intersecting at the same point without straying from the planes. Thus the structure of a garnet does not seem structurally able to display six rays. Maybe that why the six-rayed garnets are so rare? Or perhaps some kind of disconformity be causing this structural exception? Yet garnets are known for their impeccable consistency of structure. Are star garnets really garnets or some other mineral that shares the deep purple coloring of the almandine garnet? To me, at least, star garnets still remain a mystery.

A necklace made up of two garnets mined this summer at Emerald Creek, which is located in northern Idaho (Photography courtesy of Callie Modic). The bottom garnet is valued because it is a four rayed garnet (though you can't see the rays in the picture). The top garnet is valued for its color and lack of blemishes (pictured enlarged below).

Photography courtesy of Callie Modic.

Related Links

For commercial information on star garnets and Idaho ("The Gem State"):

http://www.idahogems.com/gemstone.htm

http://idahoptv.org/outdoors/shows/gems/gempics.html

http://www.minerals-n-more.com/GarnetST_Loose.html

For pictures of garnets:

http://www.idahogems.com/gemstone.htm

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

For more detailed chemical and structural information on garnets:

http://www.sbg.ac.at/min/studies_e/studies_what_e.htm

http://www.geology.wisc.edu/gems/Lect15.html

For information on where to find specific varieties of garnets in the United States, go to http://www.usgs.gov and serch for information on garnets.

Garnet with clearly formed faces still attached to a piece of rock (Photography courtesy of Callie Modic).

The surface of this garnet has been rusted, yet the dodecahedron structure of the crystal faces remains distinct (Photography courtesy of Callie Modic).

The Structure and Uses of the Mineral Garnet

Introduction

Today, geologists estimate that there are 3,000 to 5,000 identified minerals in the world. Garnets stand out from their fellow minerals in several ways. The brilliant mineral is much more abundant than most gems. This quantity is at least partially due to the mineral's ability to produce strong, well-formed crystals. Because garnets form in high pressure and temperature, they are often used an index mineral for identifying metamorphic grade. Garnets are play a significant industrial role as an abrasive. Yet the mineral still remains a beautiful and affordable gemstone. As a semiprecious stone, garnets are valued for their vast array of colors (they can be any color except blue), their hardness, and their well-defined crystal structure.

Where Garnets are Found

Though they can be found in sedimentary rocks, garnets are typically found in rocks that have experienced the high temperature and pressures of metamorphic and even igneous rocks. As such, garnets are used as an index mineral in identifying the metamorphic grade of outcrops.

Well developed garnets attached to a metamorphic rock. (Courtesy of Ron Parker at http://www.earlham.edu/~parkero/GEOS350/Magnet%20Cove/page_05.htm).

The Structure of Garnets

Garnets are classified as silicate minerals because silicate is common to the structural formula of every variety of garnet. The silicate is bonded to two groups of metals. The general formula for garnets is A3B2(SiO4)3. In this formula, the A group stands for calcium, iron, magnesium and/or manganese, and the B group represents aluminum, chromium, iron and/or manganese (less common garnets possess vanadium, titanium, zirconium and/or silicon). Differences in the metals of the A and B groups account for variations in garnet color, refraction, and density.

However, differences in various garnets structural formulas do not account for differences in the atomic structure, for all garnets possess almost identical atomic structures. Garnets are also isostructural, which means that they all possess the same crystal structure. Garnets are classified as isometric crystals. The isomeric crystal class is defined by very symmetrical, cube-based crystals. However, the most dominate crystal form for a garnet to take is the dodecahedron, a twelve sided crystal with diamond-shaped (rhombic) faces (For a diagram of a dodecahedron, go to http://www.geology.wisc.edu/gems/jpeg/l15s26.jpeg). The dodecahedron is the symbol of the garnet, and no other crystal formation is as closely associated with a mineral group.

Because of a combination of its very dominate crystal structure and the mineral's general hardness, garnets tend not to allow their surrounding area to impede their development. Consequently, garnets usually possess well-shaped, complex, and completely formed crystal faces.

Two photographs of different perspectives of the same garnet. In the photographs, we can clearly see the garnet's well-defined dodecahedron structure (Courtesy of Ron Parker at http://www.earlham.edu/~parkero/).

Types of Garnets

Pyrope is recognized because of its dark, blood-red color. It is the most recognized form of garnets. Its name s rooted in the Greek word for fire. Pyrope varies in color from a rose-red to violet variety, and its stones are frequently completely free of inclusions.

Almandine is the most common form of the mineral garnet. This abundance in combination with almandine's tendency to be black and opaque result in inability to be used as a gemstone. Rather, almandine is commonly used as an industrial garnet. The backs of those stones light enough to be gems are often hollowed out by jewelers in order to let more enter the stone. This variety of garnet is also know for its four-rayed stars.

An almandine garnet (Courtesy of http://www.idahogems.com/gemstone.htm ).

Spessartine in its most natural form is know for its orange brilliance. However, iron impurities are present in most stones resulting in a more red or brown tint. Pink is also a possible color. Because spessartine lacks in distinction and quantity in comparison to the other forms of garnet, the variety remains relatively unknown and unpopular.

Three spessartine garnets (Courtesy of http://www.idahogems.com/gemstone.htm ).

The grossular garnet's named is rooted in the Latin name for the pale green gooseberry which the mineral tends to resemble. While the light green shade is the most popular of the grossular garnets, its color can range from pink, brown, or yellow. The green form of this garnet has been mistaken for jade in the past. As such, it is sometimes referred to as South African Jade or Transvaal Jade. One particular form of grossular garnet is sometimes used in carvings and sculptures.

Three browish-yellow grossular garnets (Courtesy of http://www.idahogems.com/gemstone.htm ).

The more popular green shaded glossular garnet

(Photography courtesy of Callie Modic).

Because of its rarity, the demantoid is the most valuable variety of garnet. Its fiery color made it very popular in the 19th century. However, this popularity decreased because of the demantoid's rarity, relative softness, and small size. Demantoid is commonly known for its inclusions, which resemble small and subtle horsetails.

Uvarovite is know for its emerald green color and small crystal size. This is the rarest form of garnet. It is rarely uses as a gemstone because of its rarity and lack of size. More often it is used only as a mineral special for collectors.

Garnets as Index Minerals

When studying metamorphic zones, geologists map out the areas according the metamorphic grade of the outcrops. These zones are defined by the index minerals that they contain. Index minerals are minerals that geologists have discovered to be reliable markers for a specific metamorphic grade. As the temperature of the metamorphism increases, shale produces chlorite, metamorphic biotite, and then metamorphic garnet, straurolite, and then sillimanite. Because garnet is a index mineral for shale that has undergone a medium grade of metamorphism, we can assume that rocks with garnets in them were formed under high temperature and pressure.

Uses of Garnets: Industrial vs. Commercial

Garnets have been used as abrasives since prehistoric times. They are suitable for this function due to their relatively high hardness and lack of cleavage. In 1878, Henry Hudson Barton (founder of Barton Mines Corp.) was the first person to use garnets for industrial uses in the form of garnet coated sandpaper. This introduction rapidly grew into the extensive use of industrial garnets worldwide. In the present day, the yearly use of industrial garnets has climbed to be more than 110,000 tons per year. Despite common belief, the United State's investment in industrial garnets far outstrips its interest in garnets as semiprecious gems, for in 1994, United States valued its garnet gem production at $233,000 while its industrial garnet production was valued at about $14 million.

A display of the various shapes and sizes of cut and polished commercial garnets. (Courtesy of http://www.idahogems.com/gemstone.htm ).

As a gemstone, garnets display all the desired characteristics of other valuable gemstones. Garnets are hard and do not have cleavage, which makes them durable. Moreover, they refract light very well and can be any color except blue. Despite these qualities, garnets are not proportionately valued to similar gemstones. The source of this devaluation may be garnet's abundant quantities and widespread industrial use, for both drive down the price of the gemstone. However, the gemstone form of the mineral still retains popularity due to their availability and lower price.

Literature Cited

Johnsen, Ole. 2000. Minerals of the World. Princeton. Princeton University Press.
Marshak, Stephen. 2004. Essentials of Geology. New York. W. W. Norton & Company.
Perkins, Dexter. 1998. Mineralogy. Upper Saddle River. Prentice Hall.

http://www.earlham.edu/~parkero/GEOS350/Magnet%20Cove/page_05.htm

http://www.idahogems.com/gemstone.htm

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

http://www.geology.wisc.edu/gems/Lect15.html

http://www.minerals.net/gemstone/gemstone/garnet/garnet.htm

http://www.minerals-n-more.com/GarnetST_Loose.html

http://minerals.usgs.gov/minerals/pubs/commodity/gemstones/sp14-95/garnet.html

http://idahoptv.org/outdoors/shows/gems/gempics.html

http://www.sbg.ac.at/min/studies_e/studies_what_e.htm

Link to other Student Webpages for 2005 Earlham Physical Geology

Copyright © 2005 Earlham College. Revised April 23, 2005 . Send corrections or comments to Callie Modic