|Chromian diopside (emerald green) with pyrope and almandine garnet collected from anthills in the |
Green River Basin of Wyoming. One pyrope(center), was faceted.
|Artist's cartoon of a kimberlite|
pipe in cross section. When such
volcanoes contain diamond, they
can be worth $billions of dollars.
More than 100 such pipes and dikes
have been found in Colorado &
Wyoming with diamonds and
These eruptions have been liken to a shotgun blast shot from the earth. Many xenolith rocks in these volcanoes are shot from the mantle originating at depths of 90 to 120 miles. When erupted, most of these rocks are rounded. Others are plucked from the wall rocks at the surface all the way down to the mantle. Angular blocks and boulders are down-dropped in the magma from the surface in the rapidly cooling magma. At the Sloan kimberlite in Colorado, the GemHunter was able to examine blocks of rock that were the size of a golf cart in the rib of the Sloan 2 underground mine. These are common in many kimberlite pipes.
|The maar volcano at the Maxwell kimberlite (Colorado) shows a |
distinct shallow depression with different vegetation than the
surrounding granitic country rock.
If you were standing near a kimberlite eruption, you would hear a sonic boom! The gas from this magma being under incredible pressure erupts at Mach 2 to Mach 3. Remember those jets in years past when they broke the sound barrier – they were many miles away! Not only might this destroy your ear drums, you would likely suffocate because of all of the carbon dioxide pushing away oxygen - if you were a tree, you would likely enjoy the added CO2. Even though CO2 is harmless (in spite of the EPA) in normal concentrations, such massive amounts would push all oxygen out of the immediate area leaving you with no air to breath. Now if somehow this didn’t kill you, you would have to avoid cannon balls of mantle nodules and diamonds shot from the volcano – these would be like BBs from a shotgun blast.
|Chromian diopside collected from Schaffer kimberlites in|
Wyoming showing the characteristic box-like crystal habit
of diopside and the emerald green color of chrome.
In the Colorado-Montana-Wyoming region, there are many kimberlites, lamproites and lamprophyres. Nearly all that have been tested contain diamond. Wouldn’t you think that the States would investigate these resources. If they would have, they might not be facing bankruptcy. Some diamonds have sold for more than 200,000 times an equivalent weight in gold! Imagine what a few dozen diamond mines would do for the economy and tax base.
In the past, research for these kinds of projects at the Wyoming Geological Survey were poorly funded and often measured in the level of a few hundred dollars/year to search for gemstones, precious and base metals as well as conduct regional geological mapping projects, lectures, write publications, assist prospectors and companies, etc. Yet with this poverty level of funding we found evidence of a major diamond province. In Canada, each kimberlite discovery is estimated to cost a minimum of $1.5 million. In Colorado and Wyoming, more than 100 kimberlites were discovered on a budget of about $30,000 over 30 years! Now imagine what $1.5 million would have done! One could write a book about this!
|In this aerial photo, large cryptovolcanic structures occur in a field of >50 probable kimberlite|
pipes I discovered a few years ago. Several million people drive by these every year not paying
realizing they are driving next to some probable diamond mines along I-80! The white color in the
depressions is calcium carbonate. Due to CO2 gas, much of it becomes fixed as calcium carbonate
upon cooling. Country rock surrounding these depressions is dominated by silicate minerals with
no obvious source of calcium carbonate. Over the past few years, nine similar districts that include
a few hundred similar cryptovolcanic structures – nearly all are unexplored were found.
The gemstone is referred to by its mineralogical name (chromian diopside), but it has also been referred to as chrome diopside, chromian pyroxene, and as Cape Emerald, a misnomer after the first cut specimens from Cape Town, South Africa. Since the term Cape Emerald was first applied to gem-quality prehnite, to avoid confusion, it would be more appropriate to refer to chromian diopside as ‘Northern Cape Emerald’ based on its type locality at Kimberley in the Northern Cape Province of South Africa, where the gem was recovered from diamondiferous kimberlite along with Cape Ruby (pyrope garnet) (see the November, 2009 GemHunter newsletter) and of course, diamond.
|Emerald green chromian diopside surrounded by pyrope garnets. This|
material was collected from anthills in Butcherknife Draw south of Green
River, Wyoming and later faceted in Sri Lanka.
Chromian diopside and enstatite have comparable green color to emerald and tsavorite garnet. The primary source for chromian diopside gemstones are diamondiferous kimberlites from Siberia, where they are recovered only during warm summer months. Minor amounts are recovered by collectors and entrepreneurs in Myanmar, Pakistan, South Africa, Brazil, Italy, North America, Sri Lanka and Finland. It is a relatively inexpensive gem due to low hardness and because of rarity. Being so rare, there is little effort to market the gemstone - and with gemstones, marketing is everything. Thus the small numbers of chromian diopsides that make their way to market sell for $50 to $200/carat for faceted stones. Faceted chromian diopside gems >2 carats are rare.
|Parcel of gemstones from the Sloan 1 and 2 kimberlites (Colorado) that includes pyrope garnet |
(purple to red) spessartine garnet (orange) and high-quality transparent emerald-green
Because of relatively high specific gravity, chromian diopside can be panned using a gold pan and is recovered with black sands. During the 1980s, I had a Federal grant that we used to collect >1,600 panned samples to search for hidden diamond deposits (kimberlite). About 300 'kimberlitic indicator mineral anomalies' were identified in the Laramie and Medicine Bow Mountains (Wyoming), some of which included chromian diopside. This was a 20% success rate and indicates that a major swarm of undiscovered kimberlites occurs in these areas (http://diamondprospector.webs.com). Essentially all of these remain unevaluated.
|Kyanite eclogite nodule from the Aultman 2 kimberlite, Wyoming. Note|
the green chrome diopside, gem-quality blue kyanite, and glassy garnet.
Age dates on the intrusives (and diamondiferous host rocks) in the Wyoming Craton tell us that kimberlite (and related magmas) erupted in multiple episodes that included: (1) prior to 2 billion years ago, (2) at the end of the Precambrian (~ 600 million years ago), (3) in Late Silurian to Early Devonian (400 to 420 million years ago), (4) the Cretaceous (140 to 70 million years ago), (5) the Tertiary (60 to 30 million years ago) and the (6) Pleistocene-Quaternary (3 to 1 million years ago).
|A 2-inch long chromian diopside megacryst in kimberlite from the Sloan pipe, Colorado.|
The kimberlitic magmas (as well as some lamproites and lamprophyres) acted as transporting medium for mantle rocks containing chrome diopside and individual chrome diopside megacrysts that were trapped in the magma. Such mantle rocks as pyroxenite, dunite, eclogite, lherzolite, wehrlite and harzburgite were trapped in the kimberlites – these are known as nodules or ‘xenoliths’ (foreign rock fragments).
Some of the cognate crystals and xenocrysts found in kimberlite, lamproite and lamprophyre include diamond and other rare minerals. Since these are associated with kimberlite (kimberlite is one a few magmas that originates deep enough to sample these) they have become known as ‘kimberlitic indicator minerals’ in the diamond exploration industry. The kimberlitic indicator mineral suite includes chromian diopside, chromian enstatite, pyrope garnet, picroilmenite, chromite and diamond. When kimberlites are found with sufficient quantities of diamond, it may be mined for diamond. The other gemstones, chromian diopside and pyrope garnet that could provide value-added gems to the mine, are in nearly every case ignored by mine operators due to their lower value and possibly due to ignorance. However, recovering these gems and marketing them effectively would provide added value. As it stands, only collectors who gain access to mine tailings, recover these other gemstones usually by hand sorting from the mine tailings.
|Facet-quality chromian diopside found in both the State Line district (Colorado-Wyoming) and in the Green River Basin (Wyoming).|
Some specimens collected in the State Line district have included chromian diopside megacrysts up to 2 inches across, impressive pyrope-almandine megacrysts up to 5 inches in diameter, and eclogite and peridotite (lherzolite) cobbles (filled with chromian diopside) up to 1.5 feet in diameter. One eclogite nodule recovered from the Sloan kimberlite contained 20% diamond! At the Iron Mountain district (Wyoming), several kimberlites have chromian diopside, but not in as great quantity as those in the State Line district.
Useful maps for the State Line district include: (1) US Geological Survey topographic maps and US Bureau of Land Management Surface and Mineral Management Status maps of Ft. Collins and Laramie (1:100,000 scale); (2) General location maps of known kimberlites (see Hausel, 1998); (3) Preliminary (1:24,000 scale) geological map of the Wyoming portion of the Colorado-Wyoming State Line district (see Hausel and others, 1981). For the Green River Basin, see (1) US Geological Survey topographic maps of Firehole Canyon and Evanston (1:100,000 scale).
Elsewhere in the US, chromian diopside has been found in (1) serpentinized breccia, kimberlite and anthills at Buell Park and Garnet Ridge Arizona, (2) serpentinites in northern California; (3) kimberlites in Estes Park and the City Park of Boulder, Colorado, (4) kimberlite in Middle Sybille Creek, Wyoming, (5) the six pack lamprophyre, Wisconsin, (6) the Homestead and Williams kimberlites, Montana, (7) Green Knobs kimberlite, New Mexico; (8) Cascadilla George kimberlite, New York, (9) Winkler and several other kimberlites in northeastern Kansas, (10) kimberlites in Elliott County, Kentucky; (11) Cane Valley, Mule Ear, Mosses Rock kimberlites and serpentinized breccias, Utah; (12) Mt Horeb kimberlite, Virginia; and (13) Lake Ellen kimberlite, Michigan. And there are several other sites (see Hausel, 1998).
Several years ago, while searching for diamonds in northern California, I was searching for evidence for high pressure rocks and discovered chromian diopside in serpentinites overlying a subduction zone. In this same region, a few relatively large diamonds were found in Hayfork Creek near the Trinity River. The serpentinites were discovered near the towns of Weaverville and Hayfork.
When faceted, chromian diopside is difficult to beat. It is a beautiful stone that is as attractive if not more attractive than emerald. It is just difficult to get a consistent supply simply because no company exclusively mines for chromian diopside. It is a by-product of diamond mining and few diamond miners recover the stone as it usually reports to the mine tailings. If you watch any of the jewelry channels, you will see this stone often as supplies become available.
|Chromian diopside showing cleavage plane, collected from|
See the following two sites for more photos of chromian diopside: Cash & Treasures
Google Products and GemHunter
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