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).

Common Measures for Gemstones

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

Jade slick with weathering rind

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).

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, 28 p. text, by Wayne M. Sutherland and W. Dan Hausel (2002).

For a complete listing of WSGS materials, go to the Online Store.

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.

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