Nephrite Jade / Wyoming Jade
Nephrite jade (also known as “Wyoming Jade”), the Wyoming State Gemstone, was first described in the Granite Mountains area of central Wyoming in 1936 (Sinkankas, 1959; Sutherland, 1990). The most intense jade exploration and mining activity occurred there between about 1940 and 1960 (Madsen, 1978). Recent exploration activity indicates renewed interest in Wyoming Jade.
Gemologists apply the term jade to two distinct and unrelated mineral species: jadeite and nephrite. These two types of jade resemble each other so closely that they are essentially indistinguishable in most hand specimens isolated from their geologic environment of origin. Positive distinction between the two jades often requires the aid of tests such as petrographic, specific gravity, chemical, and/or x-ray diffraction analyses (Hausel and Sutherland, 2000). Wyoming Jade is nephrite jade; jadeite is not known to occur in Wyoming. Most of the high-quality jade found in Wyoming has been extracted from alluvial deposits in and around the Granite Mountains of central Wyoming. Jade’s specific gravity of 2.9 to 3.02 is not great enough to concentrate usable-sized pieces into well-defined placers.
Wyoming Jade is considered to be some of the finest nephrite in the world, against which material from other deposits must be compared (Sinkankas, 1959). Wyoming’s nephrite jade varies from translucent to opaque, and ranges in color from off-white (rare) to apple green, emerald green, leaf green, olive green, black, and green and white “snowflake jade” (Hausel and Sutherland, 2000). Detrital nephrite jade is found over a wide area of central Wyoming, from the southern end of the Wind River Range on the west to the Platte River near the town of Guernsey on the east, and from Sage Creek Basin along the Sierra Madre on the south to near the town of Lysite on the north.
Nephrite is an amphibole made up of extremely dense and compact fine-grained fibrous tremolite-actinolite, whereas jadeite is a pyroxene of the augite series composed of fine-grained, compact massive aggregates of interlocking crystals. The felted fiber (nephrite) or interlocking crystal (jadeite) textures of these two different minerals both result in a toughness and durability much greater than anticipated from their moderate hardness (5.0 to 6.0 for nephrite and 6.5 to 7.0 for jadeite). This moderate hardness combined with great toughness makes jade, regardless of whether it is jadeite or nephrite, relatively easy to saw and carve into delicate but extremely durable objects of art and adornment (Sinkankas, 1959).
Relief carving in thin slab of
Wyoming nephrite jade.
Backlit relief carving in thin slab
of Wyoming nephrite jade.
Because of its fibrous aggregate structure, nephrite has no cleavage. If broken, nephrite shows a brittle and irregular fracture that produces a dull to splintery fractured surface unless accompanied by inclusions of other minerals such as mica (Sinkankas, 1959). Nephrite may occasionally show a direction of separation if it has developed schistosity (Bauer, 1969).
Nephrite is a calcium magnesium silicate [Ca2(Fe,Mg)5Si8O22(OH)2] intermediate in composition between actinolite [Ca2(Fe,Mg)5Si8O22(OH)2] and tremolite [Ca2Mg5Si8O22(OH)2]. Bauer (1969) provided a chemical analysis of nephrite from Eastern Turkestan that shows a small amount of sodium present within the mineral (1.28 percent Na2O in that particular specimen). Madsen (1978) also noted minute amounts of sodium in some analyses of Wyoming nephrite. It is most likely that other elements occasionally substitute within the nephrite structure depending on its specific environment of formation. Harlow and Sorensen (2001) cite an uncommon emerald green color in nephrite as resulting from the presence of chromium within the crystal lattice. The typical green color of nephrite derives from the presence of iron within the crystal lattice. In the absence of iron, nephrite is almost colorless, but appears cloudy white because of its micro-fibrous structure. Black nephrite generally results from excess iron (Sinkankas, 1959).
Nephrite is associated with actinolite and tremolite as a member of the same mineral series. It is also associated with clinozoisite, pink zoisite, epidote, chlorite, white plagioclase, and mica as byproducts related to metasomatic alteration (link to glossary) of amphibolite (hornblende) to nephrite (Hausel and Sutherland, 2000). Other minerals found in association with nephrite include graphite, chromite, magnetite, calcic garnet, diopside, apatite, rutile, pyrite, datolite, vesuvianite, prehnite, talc, serpentine polymorphs, and titanite (Harlow and Sorensen, 2001). Foreign mineral inclusions within nephrite are common and may include those previously mentioned, along with various black iron minerals, micas, quartz, and, at one California location, fine specks of gold (Sinkankas, 1959).
Genesis and geology
Nephrite, a product of metasomatism and alteration, occurs as sheets, lenses, and nodules along or near contacts between dissimilar rock types in strongly metamorphosed zones (Sinkankas, 1959). It also occurs in or adjacent to faults and fault zones. Nephrite in Wyoming is found within granitic gneisses and granites where enclosing or intruding amphibolites have been altered. Sherer (1969) investigated nephrite jade in Wyoming and suggested that it developed from metasomatic alteration of amphibole during metamorphism. Disrupted blocks of amphibolite became trapped as xenoliths in quartzofeldspathic gneiss, and were subsequently altered to nephrite by fluids derived from regional amphibolite-grade metamorphism. Amphibole, primarily hornblende, reacted with hot metamorphic fluids to produce actinolite (nephrite jade), clinozoisite, and chlorite. In the Laramie Range, this alteration was concentrated along shear zones and fractures where it resulted from the emplacement of quartz diorite, vein quartz, or pegmatite. Water loss at sites of nephrite development stopped the alteration process that otherwise would have eventually converted nephrite into serpentine. Wallrock alteration accompanied nephrite development, bleaching leucocratic granite-gneiss adjacent to the jade to produce a mottled pink and white granite-gneiss halo with associated secondary clinozoisite, pink zoisite, epidote, chlorite, and white plagioclase pervasively altered to mica (Hausel and Sutherland, 2000).
Once formed, nephrite jade is much more resistant to erosion than its enclosing rocks, and is often found in residual and alluvial deposits as rounded boulders and cobbles. Alluvial jade tends to be solid, flaws and weak or impure zones having been removed during erosional processes. Because of nephrite’s durability, cobbles and boulders often survive transport over great distances from their source. They may also take on a natural polish from fluvial abrasion and from wind-driven sand in desert areas such as the Granite Mountains. Naturally polished pieces of nephrite jade exhibit a high-gloss, waxy surface and are known as jade slicks (Hausel and Sutherland, 2000). When not in the form of slicks, nephrite pieces may be covered with a cream to reddish brown oxidized weathering rind that hides the jade’s true color. Experienced jade prospectors learn to recognize this weathered surface, and can often recognize jade that others have overlooked.