Nephrite Jade / Wyoming Jade
Introduction
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. |
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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.
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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. |
Physical properties
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).
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| 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).
Geochemistry
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).
Mineral associations
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.
Gemology
Nephrite quality is based on uniformity of texture and lack of inclusions, coupled with color intensity and translucency. Most nephrite contains some inclusions, which can affect its ability to take a polish (Sinkankas, 1959). Lighter and brighter colors are rarer, and are valued more highly than dark colors. Similarly, more translucent nephrite commands higher prices than nearly opaque material. The interrelationships between these factors play a key role in the overall assessment of an individual piece of nephrite jade. Finished pieces of high-quality material vary upward from a few carats to many kilograms. Only about 10 percent of most nephrite deposits are gem-quality; the remainder of the marketable material is either of medium or low quality (Barnes and others, 1987).
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Distinguishing nephrite from other materials
Minerals and rocks that have been mistaken for nephrite jade include serpentinite, amphibolite, metadiabase, leucocratic granite, epidote, and fuchsitic quartzite (Hausel and Sutherland, 2000). Nephrite jade has a waxy or greasy to vitreous luster, and a sharp-edged and splintery brittle fracture. Nephrite jade’s monoclinic fibrous crystals can only be seen under a microscope and occur in felted masses.
Nephrite’s lack of granular structure easily distinguishes it in rough pieces from rocks such as metadiabase, amphibolite, and leucocratic granite. A freshly broken surface of quartzite tends to sparkle with sunlight reflected off of individual quartz grains and conchoidal fractures, appearing notably different than jade. Epidote is distinguished from jade by its perfect cleavage and pistachio green color. Serpentinite is relatively soft, can often be scratched easily with a knife, and contains zones that are weakly to moderately magnetic – all characteristics not found in jade (Hausel and Sutherland, 2000).
Nephrite deposits
Although records are not readily available, Wyoming Jade production from primary sources appears to have exceeded that of detrital origin. In-place jade veins vary from paper-thin to greater than 10 feet.
Detrital deposits
Detrital nephrite jade, commonly known as jade float, has been most abundant in the vicinity of the Granite Mountains, where primary nephrite deposits occur in Archean rocks. Detrital nephrite is dispersed widely from that source. Detrital jade ranges in color from near-black and opaque to prized translucent apple green. Most detrital jade is a pleasing medium to dark green, but may be covered with a cream to reddish-brown oxidized weathering rind that hides its true color. Cobbles and boulders may also take on a natural polish from fluvial abrasion and from wind-driven sand. These naturally polished pieces of nephrite jade exhibit a high-gloss waxy surface – such pieces are called jade slicks (Hausel and Sutherland, 2000).
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Much of the jade float has been reworked several times by erosional processes. Detrital nephrite is found within Tertiary conglomerates, and Quaternary colluvial, elluvial, and alluvial materials. The oldest known host units for detrital nephrite are the Eocene Wind River, Wasatch, and Battle Springs formations, and the Ice Point Conglomerate. Detrital nephrite also occurs within the Oligocene White River Formation, the Miocene Split Rock Formation, and in Quaternary lag gravels (Love, 1970). Quaternary terrace deposits and alluvium host minor nephrite occurrences, sometimes at great distances from primary outcrops. Nephrite jade float has been hunted extensively for years in central Wyoming, and has been picked thoroughly from much of the Granite Mountains area where it was once abundant. Jade float is still found there, but erosion exposes only small quantities each year. |
The largest nephrite boulder reported from Wyoming weighed 14,000 pounds, consisted of low-quality black jade, and was found in the Prospect Mountains near the southern tip of the Wind River Range (Hemrich, 1975). Most alluvial jade was found on or very near the surface. As such, the potential for large resources of alluvial jade in undisturbed alluvial boulder deposits of central Wyoming is quite large. These resources could easily equal or exceed the amounts already recovered.
Primary deposits
Primary nephrite deposits are hosted by Precambrian granite-gneisses of the Granite Mountains in central Wyoming, in the Seminoe Mountains, and in the northern part of the Laramie Range in eastern Wyoming. Most known primary deposits are dark green to medium green nephrite – primary sources that produced the translucent, light apple green nephrite found in detrital material have escaped discovery except for two tiny veins. The Cenozoic history of the Granite Mountains area includes major episodes of faulting, uplift, erosion, deposition, and subsidence (Love, 1970). The active geologic processes in this area could easily have reburied the primary apple green nephrite deposits, or perhaps eroded them completely.
The Granite Mountains, cored by Archean granitic gneisses and granite interspersed with amphibolites, are a deeply eroded remnant of a formerly extensive mountain range that dominated central Wyoming during much of the Tertiary Period. Numerous small jade deposits in the Granite Mountains derive from amphibolite inclusions within Archean quartzofeldspathic and granitic gneisses that have been altered to nephrite by fluids generated during regional amphibolite-grade metamorphism (Sherer, 1969). These deposits are found throughout the Granite Mountains, but are most extensive in the Tin Cup district in the southwestern part of the area. Most primary jade deposits host medium to dark green nephrite, with only traces of apple green material. Additional areas characterized by typical alteration assemblages of clinozoisite, pink zoisite, epidote, chlorite, and white plagioclase pervasively altered to mica indicate that the Granite Mountains may host many more undiscovered primary nephrite deposits at shallow depths (Hausel and Sutherland, 2000).
Southeast of the Granite Mountains, minor black to dark green nephrite jade float is found in pediment gravels and alluvium along the north and east flanks of the Ferris and Seminoe mountain ranges. The Seminoe Mountains are a Laramide anticlinal uplift cored by an Archean greenstone belt fragment that has been intruded by granite (Hausel and Sutherland, 2000). Within the core of that range, minor primary dark olive green nephrite is found in a quartz diorite dike within quartzofeldspathic gneiss (Bishop, 1964; Sherer, 1969).
In eastern Wyoming, the Precambrian core of the north end of the Laramie Range hosts several small deposits of dark olive green nephrite. Jade there occurs within orthoamphibolite dikes intruded into Archean granites. Jade float, interpreted to originate from these sources, is found in alluvial deposits of the North Platte River as far east as Guernsey.
Recommended Reference Material
Further information about Wyoming Jade can be found in the following WSGS publications:
WSGS Bulletin 71, Gemstones and other unique rocks and minerals of Wyoming, by W. Dan Hausel and Wayne M. Sutherland, 2000, 268 p.
WSGS Bulletin 72, Minerals & Rocks of Wyoming, by W. Dan Hausel, 2005, 160 p.
WSGS Mineral Report 2002-2, Preliminary Geologic Map of the Rattlesnake Hills 1:100,000 scale Quadrangle (Link to Geologic Mapping page), 28 p. text, by Wayne M. Sutherland and W. Dan Hausel (2002).
For a complete listing of WSGS materials, go to Publications.
References
Barnes et al., 1987, World review of nephrite jade – Geology, production, and reserves: South Australia Department of Mines and Energy, Rept. Bk. No. 87/116, 48 p., 12 tables, 6 figures.
Bauer, Max, 1969, Precious Stones: Charles E. Tuttle Company, Rutland, VT & Tokyo, Japan, 647 p.
Bishop, D.T., 1964, Retrogressive metamorphism in the Seminoe Mountains, Carbon County, Wyoming: unpublished M.S. thesis, University of Wyoming, Laramie, WY, 49 p.
Harlow, G.E., and Sorensen, S.S., 2001, Jade: Occurrence and metasomatic origin – extended abstract from International Geological Congress 2000, The Australian Gemmologist, v.21, p.7-10.
Hausel, W. Dan, and Holden, Gregory S., 1978, Mineral resources of the Wind River Basin and adjacent Precambrian uplifts: Wyoming Geological Association Thirteenth Annual Field Conference Guidebook, p.303-310.
Hausel, W.D., and Sutherland, W.M., 2000, Gemstones, and other unique minerals and rocks of Wyoming – A field guide for collectors: Wyoming State Geological Survey Bulletin 71, 268 p.
Hausel, W. Dan, 2005, Minerals and rocks of Wyoming – A guide for collectors, prospectors, and rock hounds: Wyoming State Geological Survey Bulletin 72, 159 p.
Hemrich, G.I., 1975, The game warden’s jade: Gems and Minerals, no.457, p.8-15.
Love. J.D., 1970, Cenozoic geology of the Granite Mountains area, central Wyoming: USGS Professional Paper 495-C, p. C1-C154, map scale 1:125,000.
Madsen, Michael E., 1978, Nephrite occurrences in the Granite Mountains region of Wyoming: Wyoming Geological Association Thirteenth Annual Field Conference Guidebook, p.393-397.
Sherer, R.L., 1969, Nephrite deposits of the Granite, Seminoe, and Laramie mountains, Wyoming: unpublished Ph.D. dissertation, University of Wyoming, Laramie, WY, 194 p.
Sinkankas, John, 1959, Gemstones of North America: Van Nostrand Company, Inc., New York, 675 p.
Sutherland, Wayne M., 1990, Gemstones, lapidary materials, and geologic collectibles in Wyoming: Wyoming State Geological Open File Report 90-9, 53 p.
Sutherland, Wayne M., and Hausel, W. Dan, 2002, Preliminary geologic map of the Rattlesnake Hills 1:100,000 scale Quadrangle: Wyoming State Geological Survey Mineral Report 2002-2, 2 Plates, Text 28 p.
Ward, Fred, 1999, World jade resources: Arts in Asia, v.29, no.1, p.68-71, in Gemological Abstracts, Gems and Gemology, Summer 1999, v.35, no.2, p.163.
Ward, Fred, 2001, Jade: Gem Book Publishers, Bethesda, MD, 64 p.