Gem Garnet

Garnet, a relatively common mineral in the continental crust, is most often found in metasedimentary rocks known to geologists as metapelite (alumina-rich mica schist and gneiss). Metasedimentary rocks are simply recrystallized sedimentary rocks modified through eons of geological time due to deep burial in the crust where temperatures and pressures are higher than at the earth’s surface.



Beautiful gem pyrope collected by the author
from Butcherknife Draw, SW Wyoming.
The stone was faceted in Sri Lanka.

 So how did these rocks get buried so deeply? Over tens of millions of years, they were buried under increasingly thick piles of sediments shed from nearby hills and mountains. Then it took another major geological event to transport them to the earth’s surface - uplifting these along major fault zones during a tectonic event. These rocks again saw light of day after the creation of new mountain ranges that forced them from depths of a few miles to the surface. This kind of mountain building event likely occurred over tens of thousands (or possibly millions) of years.

Even though many metasedimentary rocks no longer look like their original sedimentary rock, often they leave hints of their precursors, such as remnant sedimentary bedding, crossbedding, graded bedding, rounded crystal grains, association with other metasedimentary rocks, or just chemistry that matches the former sedimentary rock. Another way some sedimentary rocks recrystallize is by being near hot magma that partially melts the sedimentary rock. Thus, sandstone can recrystallize to quartzite, limestone to marble, clay to schist, etc.

Kimberlitic indicator minerals (gemstones) from
the  Sloan 1 and 2 diamondiferous kimberlites,
Colorado. These purple, red, yellow and pink
 stones are all high-quality gemstones. The green
are fabulous chrome diopsides. This area remains
undeveloped with a diamond reserve .

There are even very, very old metasedimentary rocks that look as if they were never recrystallized and have all of the characteristics of their sedimentary equivalents. For example, along the Snowy Range Scenic Highway in southeastern Wyoming are some spectacular outcrops. These are described in a roadside guide for the layman, and shows extraordinary glacial tills and superb warm sea stromatolites in road cuts (Hausel, 1993). These rocks are >2.5 billion years old, yet they look as if they were deposited just a few thousand years ago. The outcrops also provide examples of extreme natural environmental conditions in the geological past and provide evidence of Nature-caused climate change, long before it became a popular theme of politicians, environmentalists, and used car salesmen. 

Global warming and freezing events appear throughout the geologic record. Nature continues to change our climate, and believe it or not, climate change has been part of the earth's natural processes for the past 4.5 billion years. The same is true for the entire universe during the past 13.8 billion years. So, just get some people in India to clean up their plastic and other garbage. No matter how much money we are taxed for carbon emissions, it will do nothing of value except line the pockets of the scum in Congress and the WH who continue to take bribes! Do you think Biden is compromised? We are already taxed and much of the money goes to pay for what? Do we have a secure border? Do we have wars over seas? Does Congress continue to increase their own salaries and provide loopholes that give them every opportunity to accept bribes, pay for special health care, sexual harassment lawsuits?

So, when is enough enough? Take a look at Al Gore and his mansions. How about Obama's mansions. Do any of these politician even have green energy? Have you ever seen a stretch limo that is electric? Do you think Gore, Biden, Pelosi, or Obama care one iota for the American people? How can these people make so much money employed full time by the government? How does a president, who is not a millionaire when taking office, be able to afford $multimillion mansions in Hawaii and Martha's Vineyard at sea level (do you think the Obamas fear rising sea levels?). 

Beautiful garnets found in anthills in the
Green River Basin of Wyoming.

Back to garnet. Garnets found in schists and gneisses occur as porphyroblasts. Porphyroblasts are simply distinct, individual large crystals in a rock matrix formed of smaller crystals. Garnets are equidimensional or what mineralogists refer to as isometric. Their common crystal habit is that of a dodecahedron, trapezohedron, or one that exhibits both characteristics and referred to as dodecahedron-trapezohedron (see below). Few minerals are likely to be mistaken for garnet other than ruby and sapphire. However, on close inspection of ruby (see: http://wyruby.blogspot.com), one will note it is hexagonal (six-sided) with distinct parallel planes known as cleavage (neither found in garnet). 

Even though garnets are relatively common, good quality gem material is not so easy to come by. Most garnets are cloudy, have mineral inclusions rendering them unattractive for gemstones, or are highly fractured. I have been fascinated by counter tops of polished gneiss in some hotels and a few homes that contain gem quality, rhodolite garnet. I wonder if the manufacturer of this stone realizes their counter tops are filled will of dozens of gemstones. It is all that I can do to keep myself from searching for my rock hammer – something the hotel manager would likely frown upon. Somewhere in the world, someone is mining slabs of this material for table tops, counter tops and tiles and they have no idea that in the soil down slope is likely filled with gemstones.

Wow! Now here is a beautiful pyrope - from
Green River Basin.

I’ve found this is not unusual. Many mining companies get so focused on the forest that they miss all of the trees. Several years ago, Echo Bay, a Canadian mining company hired me to find a diamond deposit in the US. While searching, I came across a small company mining a rock geologists call „olivine lamproite‟. This rock is related to kimberlite. Most olivine lamproites are diamondiferous. Examples of olivine lamproites mined for diamonds include the famous Diamond State Park in Murfreesboro, Arkansas, the Ellendale field in Western Australia, the Majhgawan lamproite in India, some olivine lamproites in Canada, Africa, China, Russia and of course, the richest diamond deposit ever found – the Argyle lamproite in Australia (Erlich and Hausel, 2000; Hausel, 2006a, 2008). 

Large chromian diopside surrounded by faceted
garnets from Wyoming

Anyway, this company was mining olivine lamproite for potassium to use as fertilizer. I asked politely if I could take some samples to test for diamonds. Their response was a rude – “blanketly blank no! we‟re mining fertilizer; there are no diamonds here”. I tried to explain about the connection between diamonds and olivine lamproite but they assured me there were no diamonds in their rock (even though they had never tested it for diamonds). Maybe there were no diamonds in the rock, but these people needed fertilizer for their brains – gem diamonds are worth considerably more than fertilizer and it is possible that somewhere in the Midwest, someone has fertilized their garden with diamonds from Kansas. 

Another example is lamproites near Rock Springs, Wyoming. I was able to recover some nice olivine from a few of these (see GemHunter – the Prospector‟s Newsletter, v. 1, no. 4, 2009) and also recovered a mineral known as chromite. The chemistry of the chromites was identical to chromites found inside of some diamonds elsewhere (Coopersmith and others, 2003). This suggests that these chromites in the Leucite Hills of Wyoming formed at the same depth where diamonds form and that the likelihood of diamonds in this region is good. Yet, I could never get the bureaucrats in the Wyoming Geological Survey (WGS) management to support any research on diamonds in this area (Hausel, 2006b). It makes one wonder what the management does?

Spessartine garnet from Wyoming.

Some garnets found in the world produce extraordinary gemstones that are unmatched in beauty. The mineral is a common accessory in many micaceous metamorphic rocks (schists and gneisses). And for those of you who hunt for diamonds (http://diamondprospector.webs.com), a variety of gem and industrial garnets are found in kimberlites: some that are several inches across. In 1995 or 1996, I visited the Kelsey Lake diamond mine in Colorado for Echo Bay. The Kelsey Lake people were mining kimberlite for diamonds. As we walked by the tailings from the mill, I picked up a handful of material to examine with a 10 power hand lens. I was curious to see if there were any gem-quality chromian diopsides (sometimes referred to as Cape Emerald), pyrope garnets (sometimes referred to as Cape Ruby), spessartine garnets and almandine garnets. The tailings were filled with gem-quality garnets and I asked if I could take a small sample to study. The mine manager would not allow me to take even a vial of material which puzzled me since they were just throwing it away. A few years later I found out why. The company had mill problems and they had lost more diamonds to the tailings than they were recovering. One company later tested these tailings: the first sample they processed of the waste contained diamonds including a 6.2 carat diamond! 

Pyropes mined by ants from Butcherknife Draw
area of the Green River Basin, Wyoming.

When found in metamorphic or pegmatite rocks, garnets typically range from millimeter-size to large crystals of 5 to 6 inches in diameter. Many of these are reddish brown to brown and mostly opaque. The largest garnets I ever found were in kimberlite and pegmatite. But such large garnets were not attractive other than for mineral collectors.

Faceted gem quality garnets vary in price based on weight and quality. One can find garnets for a few dollars a carat to a few hundred dollars for excellent stones of 3 to 10 carats. Usually, larger stones of many carats are not very attractive as they tend to have less clarity than the smaller stones.

There are six pure end-member garnet subspecies. These vary in color, specific gravity, chemistry and index of refraction and include pyrope [Mg3Al2(SiO4)3], almandine [Fe3Al2(SiO4)3], spessartine [Mn3Al2(SiO4)3], grossularite [Ca3Al2(SiO4)3], andradite [Ca3Fe2(SiO4)3] and uvarovite [Ca3Cr2(SiO4)3]. Garnets in nature form solid solutions or mixtures of end members: pure end member compositions are uncommon. Thus, garnets are often described as solid-solutions.



Gem garnets in schist from Alaska.

Some garnet species of intermediate composition include 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 relatively high specific gravity (for example, quartz is only 2.87) results in garnets reporting to the heavy black sand concentrates in stream deposits and also in gold pans. Cloudy to opaque garnets are used for abrasives. Transparent to translucent garnets are used as semiprecious gemstones.

Faceted pyrope garnet from Butcherknife Draw, 

Green River Basin. The garnet has the distinct 

purplish color of pyrope (a jeweler would likely 

call this rhodolite).

Pyrope garnet is purple-red, but may also be yellow orange when it has spessartine in solid solution. It has a specific gravity of 3.5 to 3.8 and primarily exhibits rounded habit with no visible crystal faces. Pyrope has relatively high magnesium and chromium content and is associated with ultramafic igneous and metamorphic rocks. Essentially all pyrope garnet in Arizona, New Mexico, Colorado, Montana and Wyoming have been xenocrysts (contaminates) in kimberlite and in related ultramafic (magnesium-rich) igneous rocks in the four corners region (Arizona-New Mexico), State Line and Iron Mountain districts (Wyoming) and Missouri Breaks (Montana), or have occurred as porphyroblasts in garnet peridotite nodules hosted by kimberlite or have been found as detrital grains in stream sediment samples and anthills. Pyropes have also been found in mafic breccia pipes (lamprophyres) along Cedar Mountain and in nearby anthills in the Butcherknife Draw area of the southern Green River Basin (Wyoming). The largest pyrope-almandine garnet found in Wyoming was about 5 inches across in kimberlite in the State Line district south of Laramie.

Garnet lherzolite nodule from the Aultman kimberlite,
Wyoming showing rounded gem-quality purple pyrope with
some green chromian diopside in serpentine matrix.

Almandine garnets are red to reddish brown and have specific gravities of 3.85 to 4.32. Almandine often exhibits good dodecahedral habit. 

Spessartine garnet is orange red, orange to yellow orange with a specific gravity of about 4.2. Garnets of this type have been described in granite pegmatite in the Eagle Rock-Happy Jack area of the southern Laramie Range and in pegmatite at Copper Mountain in the Owl Creek Mountains.

LOCALITIES

A number of garnet localities are reported. These include translucent to opaque almandine garnet with good dodecahedral habit from the Teton Mountains (Wyoming) and chlorite pseudomorphs after garnet from the Sierra Madre near Encampment.



Chlorite pseudomorph after almandine from Oldman property,
Wyoming. Note the distinct dodecahedral crystal habit.

Oldman Prospect. These pseudomorphs exhibit excellent dodecahedral habit, are opaque, and completely to nearly completely replaced by chlorite mica, even though they retain excellent dodecahedral crystal habit of the garnet that they replaced. This deposit, known as the Oldman, contains 1 to 4-inch diameter crystals in chlorite schist in the NE section 14, T14N, R84W. The deposit is south of Encampment, Wyoming along the Copper Creek road. It 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 excellent garnet pseudomorphs have been collected from this area. The interior of many retain primary, reddish-brown almandine garnet. For collectors, I highly recommend visiting this site.

Large pyrope-almandine megacryst from Schaeffer kimberlite complex, Wyoming. This specimen is about 4 inches across. Being highly fractured but translucent, I decided to see if this would produce
 any gem material, so I had a piece fashioned into a cabochon. The cabochon by itself is not attractive, but potentially could be made into some interesting jewelry by a creative artisan.

Tie Siding. Some extraordinary pyrope-almandine garnet megacrysts have been found in kimberlite in the State Line district (T12N-10N, R72W) south of Laramie. Rounded megacrysts as large as 5 inches across, have been found in this region. Due to their partial assimilation in the kimberlite magma during emplacement, these garnets never exhibit crystal faces and are rounded with smooth surfaces. They are all highly fractured as a testimony to the incredible forces that brought them to the earth’s surface in a kimberlite magma that erupted at gaseous emplacement velocities as fast as 2 to 3 times the speed of sound! Essentially all of the known kimberlites lie to the west of Highway 287 and are on maps by Hausel and others (1981) and Hausel (1998). So to find these, one only needs to use the geological maps – however, access is difficult due to land owners in the area even though many are located on state lands.

In the same area near Tie Siding (section 11, T12, R72W), some pegmatite was quarried for feldspar during the 1940s east of Highway 287 in the State Line district. These contain uncommon euhedral garnet (Osterwald and others, 1966) in milky quartz. At one of quarry, about 500 feet east of Highway 287, I found a fractured, fist-size, opaque, euhedral reddish-brown almandine garnets nearly 6 inches across (personal field notes, 1979). The feldspar mined from these old quarries was used to produce false teeth. Another of many examples of how mining and minerals are so important to our daily lives.

Parcel of pyrope and chromian diopsides from diamondiferous
lamprophyre at Cedar Mountain, Wyoming. Essentially
all of these stones are gem-quality and facetable.

Green River Basin. Hundreds of rounded pyrope and almandine garnets are found in breccia pipes in the Greater Green River Basin near Cedar Mountain & in anthills near Butcherknife Draw (McCandless and others, 1995). These are small, transparent, pyrope and pyrope-almandine garnets (typically <10 mm in diameter) found in anthills in association with emerald-green chromian diopside & chromian enstatite. Many produce extraordinary gemstones. In some cases, more than a hundred gems have been found in a single anthill. Others are found in the Bishop Conglomerate and elsewhere in the basin The source for these minerals are diamond-bearing lamprophyres, hundreds of which likely remain hidden (Hausel and others, 1999)!

Where diamonds, chrome diopside and pyrope come from.
Large pyrope metacryst in kimberlite from the Sloan 2 

kimberlite, Colorado.

To find this area, use Google Earth. Search for „Cedar Mountain, Green River, WY 82938‟. This will take you to the approximate location of the diamondiferous pipes and dikes. Butcherknife Draw is found simply by searching for „Butcherknife Draw, Wyoming‟. I highly recommend visiting this area, since much of the ground is in the middle of unpopulated public land. As you drive through the area, periodically stop and examine some of the anthills and look for the gems collected by ants. Many of you will wonder how these can be faceted if collected by ants. Most are actually large enough to be faceted. Many gem cutters in India and Sri Lanka specialize in cutting small gems.



Group of gem pyrope and chrome
diopsides from the Butcherknife
Draw ant hills.

Gemstones collected from anthills in the Butcherknife Draw area of the Green River Basin along with three stones that were cut from the anthill material! When you search this area, watch for diamonds – about a half-dozen have been found in the anthills so far.

Some collectors have faceted some of the anthill stones 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.

Two gems in an anthill in Butcherknife Draw (left), the author shows gem prospectors how to search for gemstones on a field trip to the Green River Basin (below), and some of flawless garnets and chromian diopsides surrounds a faceted garnet from Butcherknife Draw (bottom of page).

Location map showing Butcherknife Draw area (Green River Basin Kimberlite
Indicator Mineral Anomaly) in Wyoming – a great place for field trips.