Garnet

Dinah O. Shumway

The birthstone of those born in January – garnet – symbolizes grace, truth, and fidelity, and is recognizable by its deep red color as one of the most common semiprecious stones. 

Garnet is the most diverse of the gem minerals with varied chemistry resulting in a variety of colors.  And although since ancient times, garnet has been associated with the color red, it actually occurs in all colors except blue.  Garnet varieties pyrope and almandine or almandite occurs in shades of red and violet.  Grossularite can occur in pale green or in colors of orange-yellow to orange brown.  Varieties tsavorite and uvarovite occur in deep green colors.  Andradite can occur in transparent green and yellow green to black. 

Garnets have the general chemistry of the iron (Fe)-rich garnet almandine (3FeO Al2O3 3SiO2).  The variety of color comes from its chemical makeup that follows the garnet’s “solid solution” series.  In a “solid solution” series, ions (elements) of a similar size can substitute with each other in its crystal lattice.  For example, ions that can substitute for Fe in garnets include calcium (Ca), magnesium (Mg), manganese (Mn), and chromium (Cr).  Sometimes a combination of ions will substitute for both Fe and aluminum (Al) such as in the beautiful green uvarovite where Ca substitutes for Fe and Cr for Al.  The substitution of the ions in the crystal lattices also imparts specific physical properties to the crystals.  This makes some garnets not only beautiful as gems, but because of their hardiness, useful in other applications as well. 

A high melting point (1250°C), high resistance to chemical degradation, and a high resistance to physical abrasion make garnets good gemstones and very desirable as abrasives.  Garnets are used in sandpaper, abrasive wheels, polishing grains, powders, and grits.  Garnets are also used in sandblasting, water-jet cutting equipment and as tumbling (polishing) media.  Garnets occur as equidimensional grains so they can be separated and marketed by size, and when broken, as equidimensional grains with very sharp edges.  Hardness ranges from 6.5 to 7.5 (quartz is 7 on the Mohs hardness scale) so some garnets will scratch glass! Garnets are also used in no-skid surfaces because of their hardness and resistant to wear.  The Fe-rich variety, almandine, is most in demand as an abrasive.  Garnet’s crystal form, the 10-sided decahedron, hardness, and high density also make it suitable as a filtering agent for fluids. 

Garnets are composed of some of the most common elements in the earth’s crust:  Al, Si, O, Fe, Ca, and Mg.  They are abundant in the metamorphic rocks, schist and gneiss, and a common accessory (secondary) mineral in some igneous rocks such as pegmatite.  The Mg-Al variety pyrope occurs in mafic (dark colored) igneous rocks such as peridotites and kimberlites.  As crystals, garnets most commonly form 10-sided grains ranging form millimeters to several centimeters.  A high specific gravity and resistance to wear means that garnets are often found somewhat intact in stream deposits and are sometimes mined by dredges of drag lines during placer operations.

The United States is one of the largest produces of garnet, supplying 25 to 30% of the world production – preliminary 199 world production is about 214,000 tons.  Most of the US production comes from the Adirondack region of New York; other producing states are Idaho, Maine, Montana, New Hampshire, North Carolina, and Oregon.  Other worldwide major producers are Australia, India, and China.  Minor production of garnet comes from Russia, Turkey, Norway, Canada, Czech Republic, Pakistan, and Ukraine.