Chromian Diopside and Chromian Enstatite, both pyroxenes, typically produced extraordinary gemstones in which most emeralds pale in comparison. Yet, few gemologists have 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 in relatively large quantities. 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 of mills, 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 relatively durable gemstones.
Photos - (Top left) faceted chromian diopside from Russia. Above right - group of kimberlitic indicator minerals including a variety of pyrope & transparent chromian diopside from the Sloan 1 & 2 kimberlites, Colorado (all are excellent transparent gemstones). Bottom left - 0.5 carat chromian diopside from Colorado.
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. A rare diopside, known as chromian diopside, forms distinct, emerald-green diopside with inclined cleavage. Trace amounts of chrome (up to 2%) substitute for calcium in chromian diopside, giving it a distinct emerald-green color that rival any emerald. Chromian diopside is found in kimberlite (or related rare mantle-derived, ultramafic intrusives). In Wyoming, I found chromian diopside in kimberlite in the State Line district south of Laramie, in the 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 kimberlites in Riley County, Kansas, in a lamprophyre at Ming Bar in Montana, and in several serpentinites in northern California in the vicinity of Hayfork Creek. By far the better gemstones have been found in 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.
Above - kyanite eclogite from Aultman 2 kimberlite, Wyoming. Middle - chromian diopside magacryst from Sloan 2 kimberlite, Colorado. Bottom - Cape Rubies (pyrope garnets) & Cape Emeralds (chromian diopside) from anthills Butcherknife Draw, Wyoming.