JADE - Green Gold

Jade has produced treasures of incredible value, such as a small, simple jadeite ring that sold for more than US$2.4 million and a 27-bead emerald green jadeite necklace sold in Hong Kong for US$9.3 million. In 1999, a 2-inch diameter (0.33-inch thick) jadeite bangle sold at a Christie's auction for US$2.6 million and a jadeite cabochon of 1.4 inches in length sold for US$1.74 million [1].  All jade found in Wyoming is a variety known as nephrite (an amphibole) [2,4] similar to much of the jade found in BC in Canada. Much jade in the orient is jadeite (a pyroxene). These are indistinguishable by the naked eye and require x-ray diffraction to determine which is which.  Jade has a hardness of 6 to 6.5, but it is extremely tough and carvable. A knive only has a hardness of 5.5, so one must find tools with a greater hardness than a knife or jade to carve this mineral, such as corundum or diamond. The specific gravity of jade is about 2.8 to 3.2. Pieces are often massive and have a relatively high heft and a distinct slick surface. 

In central Wyoming, jade often is associated with a distinct set of hydrothermal alteration minerals such as sericite, bleached granite, chlorite, epidote, zoiste and clinozoisite. When these minerals are found together, one often can dig (blast) in the area of alteration and find jade. Some of the more valuable specimens of jade were found in past in the 1950s in central Wyoming. However the source of this high-quality jade was never found. All of the gem-quality jade found in Wyoming occurred as detrital cobbles, boulders and pebbles in the flats surrounding Jeffrey City and Crooks Gap. The only jade found in place was in veins and replacement deposits north of Jeffrey City. However, this jade was much lower quality than the high-quality detrital jade. Somewhere in this region, there is a hidden source for some of the best jade ever found in the world.

Nephrite jade never shows external structure, except when it (rarely) pseudomorphs a crystal habit of another mineral (see photo to left). In the Granite Mountains of central Wyoming near Jeffrey City, some nephrite is found pseudomorphing quartz. Such specimens form pseudohexagonal (six-sided prisms). In other places, jade is found enclosing quartz.

Typically, nephrite jade occurs in irregular masses that lack cleavage. Microscopically, nephrite exhibits a mass of matted, intricately interwoven fibers. This form makes nephrite extremely tough and resistant to fracturing. Unless the rock has a schistose (foliated) fabric, rounded boulders of nephrite are impossible to break with a hammer. Nephrite (jade) consists of submicro­scopic, intricately interwoven, actinolite-tremolite mineral fibers that produce a massive and extremely tough gemstone. Nephrite is a result of the alteration of pyroxene in metamorphic terrains. The nephrite form of actinolite-tremolite is one of the toughest minerals known.

Nephrite can be confused with a number of minerals and special care must be taken for accurate field identification. Positive identification almost always requires testing by x-ray diffraction (XRD).

Nephrite boulder from Crooks Gap.

 The following rules of thumb are used for field identification of jade: (1) Nephrite is heavier than the average rock of the same size. (2) Nephrite cannot be scratched with an ordinary knife blade (if it scratches, it is probably serpentine or chlorite, or another similar appearing mineral). (3) Nephrite has smooth, almost waxy appearance. (4) If the end is ground off of a suspected jade speci­men, the fresh surface will not sparkle or glitter in the sun. If it sparkles, it is not jade. (5) Nephrite should not be magnetic (if it is magnetic, it is probably serpentine). (6) Nephrite is associated with a distinct alteration mineral assemblage of pink zoisite, white sericite, pistachio-green epidote, and dark-green chlorite. 

Rocks often mistaken for nephrite include fine-grained quartzite, serpentinite and epidotite. These occur in the same geologic environment as nephrite.

Serpentinite almost always has some disseminated magnetite, thus specimens are always weakly magnetic unlike jade. Many tourists are duped into buying serpentinite by rock shops in Wyoming. Serpentinite is considerably softer than jade and can be scratched with a pocketknife - jade cannot.

Black Jade.

Quartzite can be distinguished by its granular texture since it tends to sparkle on a freshly broken surface. Epido­tite is softer, like serpentine, and can also be scratched with a knife. Nearly all serpentinite will have magnetite and exhibit small areas, lenses, or specs of magnetism.

Occurrence. Nephrite occurs primarily in the Granite Mountains and has also been reported at scattered localities from the Wind River Mountains to the northern Laramie Range in a narrow east-west band that encloses the Granite Mountains north of Jeffrey City. However, we were not able to verify any occurrences in the Wind River Mountains [3].

Jade has also been found as peb­bles and boulders in alluvial fans and soils near source rocks and periodically pseudomorphs quartz. Such pseudomorphs consist of jade replacing quartz retaining the original hexagonal crystal habit.

Individual jade localities are described in a book by Hausel and Sutherland (2000). Many are found within the Tin Cup district northwest of Jeffrey City (T30N, R92-93W), and marked by old prospect pits in the granite and gneiss.

 

Left - emerald green jade.

Jade was at one time highly sought after by Wyoming prospectors. Much of the very high-quality emerald green and translucent jade was found in Tertiary conglomerates at Crooks Gap. Lower quality light-green jade was found in place to the north of Crooks Gap in the Granite Mouintains, but the source of the incredibly valuable emerald green was never found. In the 1930s and 1940s, many jade boulders of several hundred pounds were found near Jeffry City, central Wyoming.

Clockwise starting from top left - (1) typical rind found on piece of jade from the Crooks Gap-Green Mountain area (2) rare translucent jade cabochon (from the Jay Sundberg collection), (3) alteration mineral assemblage associated with jade that includes pink zoiste, green chlorite, and white (bleached) sericitic granite.

 Below, high-quality jade is very reflective!

 

REFERENCES CITED.

  1. Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.
  2. Hausel, W.D., 1986, Minerals & Rocks of Wyoming. Geological Survey of Wyoming Bulletin 66, 117 p.
  3. Hausel, W.D., and Sutherland, W.M., 2000, Gemstones & Other Unique Minerals & Rocks of Wyoming - A Field Guide for Collectors: Wyoming Geological Survey Bulletin 71, 268 p.
  4. Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.