Chrome Diopside - Cape Emerald
Chromian Diopside and Chromian Enstatite, both pyroxenes, yield extraordinary gemstones when they have good transparency and deep-green color such as seen in the group of 5 chrome diopsides in the adjacent photograph. In this photo, you will also see a variety of garnets with colors from purple, red, orange, yellow to pink. The other minerals in the photo include picroilmenite with a tan coating on the surface of the metallic mineral, one large picroilmenite, and two octahedral metallic minerals known as chromite. And yes, these are all considered gemstones and can be faceted or cabbed.
Where in the heck did I find such a variety of gems all in one place? First, this locality also has other gemstones including many diamonds and other rare rocks that include pyroxenites, dunites, peridotites and eclogites.
I collected these gems from the Sloan kimberlites when I was consulting for DiamonEx Ltd. This area has very good potential to produce diamonds along with many other gemstones. But can you imagine - essentially every diamond mining company in the world throws all of these rare rocks and gemstones away and only focuses on recovering diamonds. When I was working on this and other diamond projects for companies like Bald Mountain, DiamonEx, Western Archon, Ecobay, etc., I recommended projects and searched for additional deposits, and had hoped to convince the companies to figure out how to recover all of the gems - but then the 2008 market crash came and the economy never recovered and here we are in 2015 and still in a recession. So at the Sloan and other kimberlites in Colorado, Montana, and Wyoming there are millions of carats of colored gemstones with millions of placer diamonds and an untold amount of lode diamonds all just sitting there doing nothing.
Most emeralds pale in comparison to transparent chrome diopside, yet, few gemologists have steady access to these extraordinary gemstones. The problem is lack of exploration and development of such deposits even though they are often found in kimberlites and in rare ultramafic massifs. Kimberlites are mined exclusively for diamond, yet many contain considerably more chromian diopside (Cape Emerald) and pyrope garnet (Cape Ruby) than diamond. Diamond companies do not make any effort to recover and promote these gemstones and focus exclusively on diamonds. However, with some modifications, it may be possible to recover these gemstones (along with diamonds) at a profit. In addition, the satiated colors of the Cape Emerald and Cape Ruby could bring some extraordinary prices if marketed properly. Chromian diopside has a specific gravity of 3.2 to 3.5 (essentially the same as diamond) and can be recovered in gold pans and other specific gravity extraction methods. The gem has a hardness of 5 to 6, which is sufficient for durable gemstones.
Physical Characteristics & Crystal Habit
Pyroxene is a common rock-forming mineral of many volcanic rocks. It occurs as fine grains in basalt and gabbro; however, many andesites have large prismatic crystals. When cut parallel to the base of a well-formed crystal, pyroxene will produce square to rectangular cross section with near right-angle edges (cleavage). Whereas amphibole, which is similar and often mistaken for pyroxene, has diamond-shaped cross sections with faces intersecting at 56° and 124°. Pyroxenes occur in two crystal systems: orthorhombic (orthopyroxenes) and monoclinic (clinopyroxenes).
Clinopyroxenes (Monoclinic pyroxenes). Clinopyroxenes include: (1) Augite Ca(Mg,Fe,Al)(Al,Si)2O6, a jet black, opaque mineral with square to eight-sided cross sections found primarily in basalt and andesite and (2) Diopside CaMg(SiO3)2. Chrome diopside is a rare diopside that forms distinct, emerald-green diopside with inclined cleavage. Trace amounts of chrome (up to 2%) substitute for calcium in this diopside, giving it a distinct emerald-green color that rival any emerald.
Chromian diopside is found in kimberlite (and related rare mantle-derived, ultramafic intrusives). In Colorado and Wyoming, I found chromian diopside in kimberlite in the State Line district south of Laramie, in some Iron Mountain district kimberlites west of Chugwater, in lamprophyric breccia pipes along the southwestern edge of Cedar Mountain and in nearby anthills in the Greater Green River Basin southwest of Green River (Hausel, 1998), in numerous anthills in the Butcherknife Draw area of the Green River basin typically contain several grains of chromian diopside and some pyrope garnet, in kimberlite in Riley County, Kansas. Similar gem-quality diopside (not as chrome-rich) was also collected from a lamprophyre at Ming Bar in Montana, and chrome diopside was also collected from serpentinite in northern California in the vicinity of Hayfork Creek. By far the better gemstones have been recovered from the Sloan 1 and 2 kimberlites in the Prairie Divide area of the State Line district in Colorado, and in anthills and lamprophyres in the Cedar Mountain and Butcherknife Draw areas of southwestern Wyoming.
Orthopyroxenes (Orthorhombic pyroxenes) include enstatite Mg2(SiO3)2 typically as tiny, black, prismatic translucent to opaque crystals in some basalts. It has well developed cleavage intersecting at 87° and 93°. A rare form of dark, emerald green enstatite, known as chromian enstatite, has been found in the Butcherknife Draw and Cedar Mountain area of the Greater Green River Basin. In this region, the mineral is found in anthills with chromian diopside, pyrope garnet, and rare diamond, and has also been recovered from breccia pipes along the southwestern margin of Cedar Mountain. The enstatite is typically a darker sea green.
Some interesting pyroxene crystals are the extremely rare, dark green, salitic pyroxene [CaFeMg(SiO3)2] in some lamproites in the Leucite Hills in southwestern Wyoming. These form tiny, 1 to 2.5 mm crystals often enclosed by diopside rims. Some other salitic pyroxenes have been recovered from pyroxenite xenoliths in the lamproites. Mitchell and Bergmann (1991) report that the Leucite Hills lamproites have the only known paragenesis of this type in the world.