Garnet

The role garnet plays in your life in part depends on your interests and hobbies. For those with an interest in gems and jewelry, several commonly known gemstones are simply just different-colored varieties of garnet. In contrast, woodworkers, model enthusiasts, and home repair aficionados know garnet better as the ‘sand’ of sandpaper. Both uses are tied to garnet’s remarkable hardness.

Description and Identifying Characteristics

The word ‘garnet’ comes from the Latin word granatus, which means ‘seed-like,’ a reference to the common appearance of garnets as discrete small red rounded crystals that look like pomegranate seeds embedded in rock. The different garnet varieties comprise a group of minerals that all share a similar crystal structure, but whose chemical composition varies as different ions substitute for one another in that crystal structure. Most garnet crystals are actually mixtures of different garnet varieties rather than being composed solely of a single variety. As a result, there is little reason for non-specialists to worry about identifying individual garnet varieties.

Although red garnets are the most common variety, garnets occur in nearly every color except for blue. This variability in color results from the garnet crystals’ transparent to translucent nature, which allows minor impurities to greatly affect its appearance. Despite their variable appearance, garnets are usually easy to identify by their hardness, crystal habit and occurrence in metamorphic rock. Garnets usually form at high temperature and pressure, so they typically occur in their crystal form as rounded dodecahedrons (twelve-sided) or twenty-four sided trapezohedrons.

Iron can substitute relatively easily for either magnesium or manganese in the garnet structure, but calcium is a larger ion and does not substitute as freely for iron, magnesium or manganese. As a result, the garnet group can be subdivided into two series of minerals – an aluminum silicate series in which Fe+2 Mg+2 and Mn+2 freely substitute for one another and a calcium silicate series in which Cr+3, Al+3 and Fe+3 substitute for one another. Natural garnets are almost always a combination of these end-members, rather than being a pure sample of one composition.

Garnets occur in some igneous rocks and pegmatites, but are more typical of high-grade metamorphic rocks. Although identifying individual varieties of garnet is not important to a non-specialist, geologists use these different varieties as a way to gauge a rock’s metamorphic grade (the degree to which it has been altered), as the different garnets’ formation and composition reflects the temperature and pressure conditions their host rock endured. Hence garnets tend to record the geologic history of their host rock.

Almandine, the iron-rich garnet variety, commonly forms from regional metamorphism of clay sediments and is the most common garnet in schists and gneisses. It also occurs in silica-rich igneous rocks, such as granite, rhyolite, or associated pegmatites. Pyrope, the magnesium-rich variety is more common in mafic igneous rocks and metamorphosed mafic igneous rocks, while the calcium-rich garnets, Grossular and Andradite, are commonly found in metamorphosed carbonate rocks. Spessartine is found in granite pegmatites and metamorphosed manganese-rich rocks, while Uvarovite is the rarest garnet variety, only occurring as crusts or seams in some chromium deposits.

As garnets are chemically and physically resistant they often survive the erosion of metamorphic rocks, becoming loose garnet grains. These garnet sands can be concentrated by wave and current action into ‘heavy’ mineral sand deposits.

In most metamorphic rocks, garnets are typically associated with the mica minerals and other metamorphic minerals such as staurolite, kyanite, and sillimanite. Calcium-rich garnet varieties usually form in metamorphosed carbonate rocks where they may be found with calcite, wollastonite, or metallic ore deposits. Garnet grains in sedimentary sandstones usually occur with other heavy mineral grains or resistant minerals such as quartz.

Garnets have been used as gemstones for thousands of years, and with other red gems were often collectively known as carbuncles. Garnets’ color, hardness, and transparency make them attractive stones for use in jewelry, but they are generally too common to be prized as high-quality gems. Presently, garnet’s main economic value derives from its hardness and lack of cleavage, which makes it an excellent abrasive. As an abrasive, garnet is primarily used as a loose-grain abrasive in air blasting or water jet cutting. In sand blasting operations, garnet sand is a safer alternative than the more traditional silica (quartz) sand because of the greater health risks of inhaling airborne silica dust. Fine garnet dust is used in the detailed finishing and polishing of optical lens, plate glass, and semiconductor materials. Garnet grains may also be affixed to materials as coated abrasives, and if you work with wood, you almost certainly have purchased sandpaper made of garnet grains. Because garnet is chemically stable, it is also used as a filtering medium for water purification systems or wastewater treatment plants.

Some southwestern U.S. Indian tribes were responsible for one of the most unusual historical applications of garnet, when the circular shape, hardness, and blood-red color of garnet crystals, combined with a lack of local lead sources, led to their use as bullets. A similar use was also reported on the opposite side of the world during a 1892 rebellion against British troops in India. Garnet’s common red color made a natural association with blood, and many cultures have used ground-up garnet for medicine’s associated with heart and blood diseases or as pendants or jewelry given in token of love or fidelity. Its glassy luster and transparent to translucent nature also led to folklore of garnet being used as a mystical light source, to lead the dead into the next realm or to light Noah’s way in his Ark.

Although there are no known Upper Midwest occurrences of gem-quality garnets, garnets are a common accessory component in many of the Precambrian granites, schists, and gneisses that compose the region’s underlying deep bedrock.

Garnet can usually be identified by its distinctive crystal habit and hardness, but other minerals in metamorphic rocks may, at first glance, may be confused with garnet.

Staurolite:

Staurolite is a common component of metamorphic rocks that may exhibit a reddish brown color similar to many garnets. However, staurolite crystals are elongated prisms that often occur as twinned intergrown crystals to form a distinctive cross pattern. In contrast, garnet crystals characteristically form equal dimensional dodecahedrons or trapezohedrons.

Apatite:

The shorter prismatic habit of some apatite crystals may give them a similar appearance to garnet, but apatite is softer than garnet only having a hardness of 5 on the Mohs scale of hardness. This means that it can be scratched by a metal blade, although it is nearly the same hardness as the metal, and cannot scratch glass. In contrast, garnet is much harder than most metals and will easily scratch a glass plate.

Pyrite:

Pyrite can occur as discrete blebs in a rock. As these irregular masses alter to limonite, the resulting rusty surface coating may make them look like red garnets. These minerals lack garnet's characteristic dodecahedral habit, however, and the rusty coating will yield a distinctive yellow to brown streak that should distinguish them from garnet.

Zircons:

Zircons exhibit a wide range of color varieties and reddish or brown zircons may initially be mistaken as garnets. On closer examination though, zircons characteristically occur as plate-like prisms that have a distinct tetragonal shape that contrasts with garnet's signature rounded dodecahedral (12-sided) to 24-sided trapezoidal crystal shape.

Tourmaline:

Tourmaline may exhibit a similar hardness and transparency as garnet, but usually formed more elongated crystals that are easy to distinguish from garnet's round dodecahedral habit. Short, stubby crystals of tourmaline are more difficult to distinguish from garnet, but tourmaline crystals will have a triangular cross-section, while garnet is more symmetrical.