The GemHunter

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 encloses quartz crystals.

Typically, nephrite forms irregular masses that lack cleavage. Microscopically, nephrite exhibits a mass of matted, intricately interwoven fibers actinolite-tremolite fibers. This form makes nephrite extremely tough and resistant to fracturing. Unless the rock has a schistose (foliated) fabric, rounded boulders of nephrite are essentially impossible to break with a hammer. 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 NEVER magnetic, in spite of what some suggest!

Serpentinite can be a problem for jade prospectors as well as first time buyers of jade who are periodically 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 Wyoming 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 disseminated magnetite, making the rock weakly magnetic. Jade does not have magnetite. 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, - likely was all serpentinite. When examined by the author several years ago, only serpentinized ultramafic komatiite, similar to that found at South Pass, could be found.

Greenish quartzite is sometimes mistaken for jade - but it has granular texture, and tends to sparkle in the sun, on a freshly broken surface. Epido­te is softer, like serpentine, and can also be scratched with a knife - it is also, distinctly pistachio green in the region of Wyoming's jade deposits. 

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, I was not able to verify 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.

Much of the high-quality, emerald-green and translucent jade, was found in Tertiary conglomerates at Crooks Gap in Wyomng.  In the 1930s and 1940s, many jade boulders of several hundred pounds were found near Jeffry City.

HOW TO IDENTIFY JADE

The following 'rules of thumb' are used in 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. (5) Nephrite is never magnetic. (6) Nephrite is associated with distinct alteration mineral assemblage of pink zoisite, white sericite, pistachio-green epidote, and dark-green chlorite. 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).