Chromian Diopside - A Gem That Surpasses Emerald in Every Way, Except Price.

Gemstones from diamond deposits. These beautiful gems were collected from 
diamondiferous kimberlites at the Sloan Ranch in the State Line district in Colorado.
The gems include a variety of pyrope, spessartine, and almandine garnet (red, purple,
pink, orange), green chromian diopside, black metallic picroilmenite with leucoxene
coating, and black, metallic, equi-dimensional chromite.

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.

Beryl Gemstones

"The fifth, sardonyx; the sixth, sardius; the seventh, chrysolite; the eighth, beryl; the ninth, a topaz; the tenth, a

chrysoprasus; the eleventh, a jacinth; the twelfth, an amethyst." - Revelation 21:20

VARIETY                         COLOR                                 CHROMOPHORES

Emerald                           Chrome-green                      Cr, V, Fe2+, Fe3+

Aquamarine                     Light blue to sea green         Fe2+, Fe3+

Maxixe beryl`                    Blue (fades in sunlight)        Fe2+, Fe3+

Chrysolite                         Yellow-green                         Fe2+, Fe3+

Golden beryl                     Golden-yellow                      Fe2+, Fe3+

Heliodor                            Greenish-yellow                   Fe2+, Fe3+

Morganite                         Pink-orange, pale pink          Mn2+, Mn3+

Bixbite (Red Emerald)     Dark-red                                 Mn2+, Mn3+

Goshenite (Rosterite)      Colorless                               None

Varieties of gem beryl and trace coloring agents (chromophores) responsible for color in the gemstones.

The GemHunter

Professor Hausel's Guide to Finding Gemstones

Beryl is a mineral of many gems and when gem-quality, it can occur as emerald, aquamarine or precious beryl. Beryl (Be3Al2Si6O18) forms distinct 6-sided hexagonal crystal prisms that are capped by flat, basal, pinachodial terminations such as seen in the photo to the left (specimen collected at Casper Mountain, Wyoming). The crystal habit is often distinct; thus, recognizing the crystal habit along with beryl found in granite pegmatite (very coarse-grained granite) is often all a geologist or prospector needs to verify the presence of beryl. Helidor and aquamarine are usually the more common varieties. 

Hexagonal beryl crystal, Casper Mountain
Wyoming

Gem varieties of beryl are all the same mineral: they just have trace coloring agents known as chromophores that give them different colors. So, yes, that extremely expensive emerald is the same mineral as that not so attractive helidor - the only difference (beside price) is just a little bit of chrome in the crystal structure of the emerald.

Much of the beryl found in pegmatite is greenish-yellow, translucent to opaque with moderate specific gravity (2.6 to 2.9) similar to quartz (2.7). Thus, when panning for gold, any beryl accidentally captured in a gold pan could easily be rejected (washed out) with quartz and other light materials. Thus, beryl has moderate heft - a hand-sized specimen will not feel abnormally heavy when bounced in your hand. With a hardness of 7.5 to 8 on Moh's scale similar to quartz (H=7) and topaz (H=8), it is plenty hard enough to produce long lasting gemstones. Because of its favorable hardness, beryl will yield a white streak when scratched by a knife or scraped across a streak plate (Hausel, 2009; Hausel and Sutherland, 2000).

Beryl has vitreous luster on 'fresh' surfaces and will produce conchoidal to uneven fracture. It has imperfect basal c{0001} cleavage. When found as transparent crystals, beryl is gem quality. Depending on color, beryl will yield a variety of gems. These are typically separated into three groups - aquamarine (light blue), emerald (chrome-green) and precious beryl (all other colors).

Most beryl will tend to crystallize out of felsic magma; particularly beryllium (Be) enriched rhyolite and pegmatite. The great emerald, aquamarine and precious beryl deposits of Brazil (and elsewhere) are associated with pegmatite and eluvial deposits, while some other beryl may have hydrothermal (hot fluid) or metamorphic (recrystallized) genesis. Secondary deposits are important in specific regions of the world (i.e., Brazil and Sri Lanka) where beryl has been naturally beneficiated in eluvial and colluvial deposits during erosion. So, when you are searching for beryl gemstones, look for pegmatites and check any geology literature about any igneous rocks and pegmatites in the region. If there is a mention of anomalous beryllium associated with granite, rhyolite or granite pegmatite, you are in the right area. In my recent 368-page book, A Guide to Finding Gemstones, Gold, Minerals and Rocks, I examined some very interesting areas with known Be-enriched pegmatites and described where many are located that have never been explored - any of which could yield common beryl or gemstone-beryl. So if you want to be a serious gem prospector - you should have a copy of my book. And don't forget to share: buy many additional copies for all of your friends, relatives and neighbors.

Stream placers are unfavorable for beryl because of the mineral's low specific gravity and prominent basal and rhombohedral parting. Disaggregation by abrasion will often result in disintegration of the mineral over short stream transportation distances. Thus, the more important gem-beryl deposits are secondary eluvial or colluvial deposits. If you find gem-quality beryl in your gold pan while panning, it indicates that the source of the gem is in the immediate area. 

Beryl may be bluish-green, light-yellow, emerald-green, golden-yellow, yellow-green, pink, reddish-pink, raspberry-red or colorless. Some of the more popular colored beryl gems are aquamarine, emerald, bixbite (red emerald), golden beryl, goshenite, heliodor and morganite. Rarely, beryl gemstones are colored by mineral inclusions (i.e., microscopic spinel) such as black beryl from Mozambique and Madagascar.

Of the beryl gems, emerald is often the most fragile due to uneven substitution of Cr (chromium) within the crystal lattice that produces internal flaws. Thus, most emeralds have minute fractures and zones of translucency intermixed with zones of transparency. Brightly colored beryl is often faceted in step or combined forms, whereas pale beryl is cut into brilliants to enhance luster. Some rare beryl has been found with asterism or silky luster caused by inclusions of epidote and apatite. These are cut into cabochons to display asterism or cat’s eyes (Kievlenko, 2003).

Some beryl specimens have included very large stones. These include the 6-foot long crystal of 2.5 tonnes (Bauer 1968b)! Another giant specimen was found in Albany, Maine that was described to be 27 feet long and weigh >25 tonnes (Hurlbut and Switzer 1979)!

Aquamarine and many precious beryl crystals are found in granite pegmatites. Much high-quality emerald is recovered from hydrothermal deposits that are not favorable for large gems, and most emeralds are considerably smaller than other beryl gems. Some extraordinary emeralds have been mined in Columbia, including a 1,759-carat doubly terminated emerald (Ward 2001a). Another large emerald, known as the Gachala, weighed 858-carats and measured 2 inches in length – this stone resides in the Smithsonian collection and is regarded as one of the finest emeralds.

Aquamarine is periodically found as very large crystals. One of the larger, the Papamel, was found in Minas Gerais Brazil in 1910. The specimen weighed 553,400 carats (244 lbs) at 18 inches long and 15.5 inches wide. It was so transparent that one could read through the stone from end to end. The stone was cut into many gems of total weight >100,000 carats (Sinkankas 1991)! Another large stone from Brazil was the Domo Pedro of 10,363 carats that yielded the largest cut aquamarine gemstone. Another large aquamarine was found at Anderson Ridge in the Wind River Mountains of western Wyoming by friend of mine and also a prospector by the name of Elmer Winters. The specimen weighed many thousands of carats, had excellent transparency and from what I remember, it was about 1-foot long (Hausel, 1991).

Because of large size and perfect hexagonal habit of beryl in some pegmatites, some extraordinary industrial specimens have been recovered that have no value as gems. Many of these, because of crystal perfection and size are mentioned as collector and museum specimens. For example, extraordinary specimens found at the Triple Chance mine in the Broken Hill district of Australia, were flawed, green, brown and blue-green beryl prisms up to 4 feet long and 18 inches wide. At the Lady Beryl mine, 2.5 miles south of the Triple Chance, 12 tonnes of crystals were produced, some of which were described as projecting from the pegmatite like fence posts. The largest was (3 x 9.5 feet).

HOW TO IDENTIFY BERYL

For me, the best way to identify beryl is: 

(1) by geological setting. The majority of beryl is found in pegmatites (coarse-grained igneous rocks, and mostly granite in composition). 

(2) Beryl forms distinct hexagonal (6-sided) crystals with long prims that end (terminate) with pinachodial (flat) surfaces.

(3) They are relatively hard. On the Moh's scale they rate as 7.5 to 8. So, they will scratch quartz (H=7), a steel nail (H=6.5) and also a pocket knife blade (H=5.5).

(4) They have a moderate specific gravity similar to quartz (SG=2.76), so they are difficult to retain in a gold pan while panning in a stream.

(5) They are vitreous and produce a conchoidal to irregular fracture.