Jade - Green Gold
Nephrite jade never shows external structure, except when it (rarely) pseudomorphs a crystal habit of another mineral. In the Granite Mountains of central Wyoming near Jeffrey City, some nephrite is found pseudomorphing quartz. Such specimens form pseudohexagonal (six-sided prisms) of light-green jade. 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 - and it is not magnetic!
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).
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 likely 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's not jade. (5) Nephrite is not 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. More information on the characteristics of jade and where it can still be found in Wyoming is published in a book on gemstones (Hausel, 2014).
Serpentinite can be a problem for many jade prospectors as well as first time buyers of jade who are scammed by sellers. Serpentinite is found in the jade fields of Wyoming and in British Columbia and many prospectors mistaken the rock for jade. I have seen rock collectors in the Riverton, Wyoming area sell serpentinite as high-quality jade to unsuspecting buyers - even after I pointed out to the unscrupulous individuals that the material was serpentinite. Even though serpentinite will typically have a nice green color similar to jade, it is softer and can be scratched with a pocketknife. According to Hausel (2014, 2009, 2005) and Hausel and Sutherland (2000), serpentine, which is found in the Tin Cup district of the Granite Mountains, only has a hardness of 2.5 to 5.5 and has scattered pockets and disseminations of magnetite, making the rock weakly magnetic. Jade does not contain magnetite and is non-magnetic. In the past, one deposit known as the Game Warden's jade found in the southern Wind River Mountains which caused quite a stir after it was discovered and reported to contain giant jade boulders - was probably all serpentinite. When examined by Hausel several years ago, only serpentinized ultramafic komatiite, similar to those found at South Pass, could be found.
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.
In Wyoming, 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.
Individual Wyoming jade localities are described by Hausel (2014). 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.
Jade was at one time highly sought after by prospectors in the US. Much of the very high-quality emerald green and translucent jade was found in Tertiary conglomerates at Crooks Gap in Wyomng. Lower quality light-green jade was found in place to the north of Crooks Gap in the Granite Mouintains. In the 1930s and 1940s, many jade boulders of several hundred pounds were found near Jeffry City, central Wyoming.
- Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Report MR06-1, 363 p.
- Hausel, W.D., 1986, Minerals & Rocks of Wyoming. Geological Survey of Wyoming Bulletin 66, 117 p.
- Hausel, W.D., and Sutherland, W.M., 2000, Gemstones and other unique minerals and rocks of Wyoming: Wyoming Geological Survey Bulletin 71, 268 p.
- Hausel, W.D., 2009, Gems, Minerals and Rocks of Wyoming. A Guide for Rock Hounds, Prospectors & Collectors. Booksurge, 175 p.
- Hausel, W.D., 2014, A Guide to Finding Gemstones, Gold, Minerals & Rocks: GemHunter Books, 369 p.