Professor Hausel's Guide to Finding Gemstones, Diamonds, Gold, Rocks & Minerals 

Gem Garnet

Ants, ants, ants - these little buggers bite with emphasis in Arizona and leave a mark for days. But in Wyoming, they work for a living - yes, they have jobs as miners and dig all summer long looking for gemstones to decorate their mounds just so you and I can pluck them off their anthills. But don't worry, there are plenty more for the ants to find and decorate their mounds before Christmas. 

Left- excellent pyrope garnet I picked up in Butcherknife Draw in the Green River Basin of Wyoming and had it faceted in Sri Lanka.

But where do they come from - how do the ants find them. Well, a good friend of mine, Dr. Tom McCandless, worked on a research project where he placed a group of beer bottle chips on grid patterns around an anthill to see what the little buggers did. Sure enough, when he later returned, the ants had sampled the beer bottle fragments (little alcoholics) and placed them on their anthill. So, they gather them from the surrounding soil in the Green River Basin. 

But then, aren't they too small to facet? Yes and no. The ants are too small to facet and they probably don't like be faceted, but those tiny little gems can be cut. And some are quite large. If I remember correctly, one of the larger stones I found in the Leucite Hills was a 12 to 14 mm peridot. That must have been one heck of a strong ant. Actually, they work in groups, and a large stone like that would be moved by a group of ants. After all, what else do they have to do?

There are many varieties of garnet. Some produce extraordinary and beautiful gems yet garnet is a common accessory mineral in many micaceous (almandine-amphibolite-grade) metamorphic rocks known as schist and gneiss and is also found in kimberlite, some granites and many pegmatites. When in metamorphic or pegmatitic rocks, garnets can range from millimeter-size to single crystals of 5 to 6 inches in diameter. Many are purplish-red, yellow-orange or reddish-brown in color with some of the largest garnets occurring in kimberlite and pegmatite.

Six pure end-member garnet subspecies are known to mineralogists. These vary in color, specific gravity, chemistry, and index of refraction and include: (1) pyrope [Mg3Al­2(SiO4)3], (2) almandine [Fe3Al­2(SiO4)3], (3) spessartine [Mn3Al­2(SiO4)3], (4) grossularite [Ca3Al­2(SiO4)3], (5) andradite [Ca3Fe­2(SiO4)3] and (6) uvarovite [Ca3Cr­2(SiO4)3]. Natural garnets form solid solutions or solid chemical mixtures of these end members such that pure end member compositions are uncommon. Thus, garnets are described as solid-solutions (they are hybrids). Names have been given to some garnets with intermediate compositions, such as the rose-red to purple rhodolite garnet, which has a chemical composition of 2:1 mixture of pyrope to almandine. Another intermediate variety of garnet with a composition between pyrope and almandine (1:1 mixture) is referred to as pyrope-almandine (also known as Mozambique garnet) that exhibits a striking dark orange-red to red color.

Garnets have relatively high specific gravity (3.5 to 4.3) and hardness (6.5 to 7.5). The specific gravity results in garnets showing up in heavy black sand concentrates while panning for gold.

Garnets crystallize in the isometric crystal system & have no cleavage. They are typically transparent to translucent and often exhibit well-formed dodecahedral or trapezohedral crystal habits. Garnets have been used for abrasives; although, excellent museum quality garnets are often found, and many transparent to translucent garnets with good color are used as semiprecious gemstones.

Occurrence. Garnet is a common mineral in many metamorphic terrains, particularly where rocks have been metamorphosed to amphibolite-grade facies and are aluminum-rich. Such rocks typically contain abundant black mica and may also have amphibole with porphyroblasts of garnet. Such rock safe typically found in many mountain uplifts in North America. They can also sometimes be found in what is known as skarn - limestone (and other carbonate-rich rocks) that has been heated and altered by the intrusion of hot magma. 

Localities. A number of garnet localities are known in Wyoming and many thousands remain to be reported or found worldwide. In Wyoming, these include translucent to opaque almandine garnet with good dodecahedral habit located in the Teton Mountains and chlorite pseudomorphs after garnet from the Sierra Madre Mountains near Encampment. These latter pseudomorphs exhibit excellent dodecahedral habit, are opaque, and completely to nearly completely replaced by chlorite mica, even though they retain the former dodecahedral crystal habit of the garnet. Some excellent 3 to 4-inch diameter crystals have been collected from chlorite schist at the Oldman prospect (NE section 14, T14N, R84W). The deposit located south of Encampment along the Copper Creek road forms a narrow schist (<10 feet wide) on both sides of the road about 1/2 mile south of the Oldman Ranch. The garnet-chlorite-schist crops out over a distance of approximately 2,000 feet, and has large, dodecahedral, chlorite pseudomorphs after garnet. Several 0.5 to 3-inch diameter garnet pseudomorphs have been collected from this area. The interior of many of the pseudomorphs contain primary, reddish-brown almandine garnet (Hausel, 2014).

Some extraordinary pyrope-almandine garnet megacrysts have been found in kimberlite in the State Line district (T12N-10N, R72W). Some rounded megacrysts as large as 5 inches across, have been found in this region. Due to assimilation in the kimberlite magma during emplacement, these garnets never exhibit any crystal faces and are always rounded with smooth surfaces.

In the same area, near Tie Siding (section 11, T12, R72W), some pegmatite was quarried for feldspar during the 1940s along Highway 287 east of the State Line diamond district. These contain uncommon euhedral garnet (Osterwald and others, 1966). At one of quarry, about 500 feet east of Highway 287, some fractured, fist-size, opaque, euhedral garnets were found.

Hundreds of rounded pyrope and almandine garnets were found in breccia pipes in the Greater Green River Basin near Cedar Mountain & in anthills near Butcherknife Draw. These are small, transparent, pyrope, pyrope-almandine garnets, and spessartine garnets are (typically <8 mm in diameter) found in anthills in association with emerald-green chromian diopside & chromian enstatite. Some collectors have faceted some of these to produce attractive emerald-green, yellow-orange, and reddish-purple gems. A few diamonds have been found in the area in anthills and lamprophyres. Similar garnets are found in anthills in the Bighorn basin north of Thermopolis.

Even though there are several great garnet localities in Wyoming, one of my favorite localities is in Alaska. And of course, there are garnets in Arizona, CaliforniaColorado, IdahoMontana, NevadaNew Mexico, TexasUtah, and even in the Appalachian Mountains. There are many places to hunt garnet and other gemstones in the US.

For more information on garnets - please search my blogs - Gem Garnet and Wyoming Garnets