Garnet has been a treasured gemstone for millenia, with its deep blood-red color evoking inner fire and passion. Garnet-adorned objects traditionally accompanied kings going into battle and pharaohs into the afterlife as a talisman providing protection from harm. Today garnet is honored as the birthstone for January, as a metaphysical stone for protection and commitment, and as an important industrial mineral.
Above: chest plate of carnelian, lapis, turquoise and garnet from the Middle Kingdom of Egypt, in the Metropolitan Museum of Art.
Typically garnets occur in metamorphic rocks (like schists and gneisses) and some igneous rocks, so it occurs on all continents and is relatively common. Garnet forms isometric crystals and has hardness on the Mohs scale of 6.0-7.5, depending on composition.
Garnet has the general formula X3Z2(SiO4)3 where X = Mg, Ca, Fe(II), Mn(II), and Z = Al, Fe(III), Cr(III), V(III). Other elements can substitute for X and Z as well, giving garnet a wide range of compositions. No surprise that there are 15 species of true garnet. Most people think of garnet having a deep red color; however, it can also be green, yellow, brown or orange depending on its chemical composition.
Photo below: demantoid garnet crystals from Namibia.
The most commonly encountered species of garnet are:
Andradite: Ca3Fe3+2(SiO4)3 green varieties are termed demantoid garnet
Grossular: Ca3Al2(SiO4)3 varieties include hessonite (cinnamon-stone) and tsavorite (from Kenya and Tanzania)
Rhodolite: a solid solution of almandine and pyrope, named for its pink color, rhodolite was first discovered in North Carolina by renowned mineralogist William Earl Hidden in the late 1890's
Below: "Raspberry" grossular garnet specimen, from Mexico.
- Garnet is the official state gemstone of New York, almandine of Connecticut, star garnet of Idaho, and grossular of Vermont.
- Before instrumental identification of minerals was possible, polished cabochons of garnet (and other red stones) were called carbuncles.
- The oldest garnet found (and dated) so far is about 2.6 billion years old, and the youngest about 5 million years old.
- The third largest ever discovered garnet crystal was found at the the Barton Mine, Gore Mountain, North Creek, New York. It was 3 feet in diameter and weighed an estimated 3400 pounds.
- The second largest garnet was about 7.5 feet diameter and weighed 83,000 pounds.
- One internet article reports that the largest recently discovered garnet crystal is from Australia. The report claims that it's about 200 feet in diameter and 130 feet high. It has not been removed from the mine where it was discovered by orders of the Australian government.
- Watch a fun video of large garnets being mined in New South Wales, Australia, HERE.
Garnet is widely used as an industrial abrasive due to its hardness for both dry and wet blasting applications. Next time you use sand paper, check to see if it’s little garnets stuck to the paper! A summary of the industrial uses of garnet in the United States can be found HERE.
Garnets have great scientific significance because they act as geobarometers and geothermometers for the rocks in which they occur. By carefully analyzing the chemistry of garnet, mica and other minerals that occur within a single metamorphic rock, the temperature and pressure conditions at peak metamorphism can be inferred by comparison with experimental results at known pressure and temperature. This has led to a much greater understanding of the conditions of metamorphism in continental crust.
Above: Andradite garnet from Santa Fe County, New Mexico.
Garnet in New Mexico
Garnet occurs in several counties of New Mexico. Notably, pyrope garnet occurs just north of Gallup in the Navajo volcanic field, and andradite garnet occurs in the metamorphic rocks of the Picuris Mountains near Taos, often accompanying staurolite. Andradite can also be collected in Santa Fe County. To read more, click HERE.