Wyoming Gemstones

Nephrite Jade/Wyoming Jade

The term jade is applied to two distinct and unrelated mineral species. Jadeite is composed of fine-grained, compact massive aggregates of interlocking pyroxene crystals in the augite series. Although minor jadeite has been reported in northwestern Wyoming, its occurrence has never been verified. Nephrite comprises extremely dense and compact, fine-grained, felted, fibrous amphibole crystals in the tremolite-actinolite series. Positive distinction between the two jades often requires the aid of tests such as petrographic, specific gravity, geochemical, and/or X-ray diffraction analyses.

Wyoming Jade is nephrite. Sinkankas (1959) considered Wyoming Jade to be some of the finest nephrite in the world. Nephrite is a calcium magnesium silicate varying in composition from actinolite to tremolite. Variations in chemistry are responsible for a wide range of colors extending from white to black, with rare occurrences of blue and off white. However, most common nephrite jade is of varying shades of green, such as apple, emerald, leaf, and olive. The green color is attributed to the presence of reduced iron, with varying contributions from other elements.

The translucent jade window (to the right) at the North Shore Baptist Church in Chicago, Illinois, illustrates some of the colors found in Wyoming Jade. The window is 6.5 feet high and 3.5 feet wide, and was dedicated in 1952. James L. Kraft, president of the Kraft Foods Company in Chicago and an avid collector of Wyoming Jade, made it for the church. The jade came primarily from Wyoming, with the wide variety of greens, blues, and other unusual colors of jade resulting from years of searching across the central part of the state by Mr. and Mrs. Allan Branham of Lander, Wyoming. Some of the jade also came from Alaska, Arizona, and California.

Hardness/Durability

Nephrite’s hardness, combined with its micro-felted aggregate structure, makes it relatively easy to saw and carve into delicate but extremely durable objects. This durability is a major factor behind Wyoming Jade’s high reputation. Nephrite has a moderate hardness of 5.5 to 6.5 on the Mohs scale, which has a range of 1 being the softest and 10 being the hardest. Because of its micro-fibrous aggregate structure, nephrite has no cleavage; if broken, it exhibits an irregular splintery fracture with a dull surface unless accompanied by inclusions of other minerals such as mica. Nephrite may occasionally show a direction of separation or schistosity due to mineral inclusions and may also host fine fractures related to tectonic movement. Either of these conditions drastically reduce the quality of the jade.

Quality

Jade quality is based upon uniformity of texture and lack of inclusions coupled with color intensity and translucency. Wyoming Jade varies from translucent to opaque. Most nephrite contains some inclusions, which can affect its ability to take a polish. Lighter and brighter colors are rarer and are generally valued more highly than darker colors. Similarly, more translucent nephrite commands higher prices than nearly opaque material. These factors and their interrelationships determine the overall assessment of an individual piece of 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 considered gem quality.

The durability of jade, when lacking inclusions or fractures, makes it extremely resistant to erosion. High-quality nephrite is much more resistant than either low-quality jade or the rocks that enclosed it where it formed. Most of the high-quality jade found in Wyoming has been extracted from alluvial deposits in and around the Granite Mountains of central Wyoming. In fact, detrital nephrite jade is found over a wide area extending from the southern end of the Wind River Mountains 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. Jade’s specific gravity of 2.9 to 3.02 is not great enough to concentrate into well-defined placers.

Rocks and Minerals Similar to Nephrite Jade

Several rocks and minerals have been mistaken for nephrite jade and can be distinguished by hardness, physical appearance, or X-ray diffraction. These include serpentine, serpentinite, amphibolite, metadiabase, leucocratic granite, epidote, and fuchsitic quartzite. The lack of granular structure in nephrite easily distinguishes it in rough pieces from rocks such as metadiabase, amphibolite, and leucocratic granite. A freshly broken surface of quartzite tends to sparkle in the sunlight off of individual quartz grains and conchoidal fractures, which are notably different than nephrite. Epidote has a perfect cleavage and pistachio-green color not found in nephrite. Serpentinite is relatively soft and can often be easily scratched with a knife, and contains zones that are weakly to moderately magnetic; all characteristics not found in jade.

Primary Jade Formation

Nephrite is considered to be a product of metasomatism and alteration, generally associated with gneisses, serpentinites, schists (especially hornblende schists), and metamorphosed limestones or dolomites. It occurs as sheets, lenses, and nodules along or near contacts between dissimilar rock types in strongly metamorphosed zones, and in or adjacent to faults and fault zones.

Wyoming nephrite occurs within granitic gneisses, usually in close association with intruded amphibolites, shear zones, faults, and local folds or otherwise distorted and broken rock. The rock has invariably been re-cemented by quartz, nephrite, or other minerals related to fluid movement. Wall rock alteration accompanies nephrite development, bleaching adjacent granite-gneiss and resulting in a characteristic appearance. Typically, this includes a mottled pink and white granite-gneiss with associated secondary clinozoisite, pink zoisite, epidote, chlorite, and white plagioclase pervasively altered to mica.

Sherer (1969) suggested Wyoming nephrite deposits developed from hydrous metasomatic alteration of amphibole during metamorphism. Iron, aluminum, and calcium mobilization accompanied this process to result in epidotization of surrounding country rocks, which may be extensive.

Amphibole, primarily hornblende, reacted with hot metamorphic fluids to produce actinolite-tremolite (nephrite jade), clinozoisite, and chlorite. This alteration concentrated along shear zones and fractures where it resulted from the emplacement of quartz diorite, vein quartz, or pegmatite.

Sherer (1969) hypothesized that water loss at sites of nephrite development stopped the alteration process that would have otherwise continued to convert nephrite into serpentine. Harlow and Sorensen (2001) suggested that the felted fibrous masses characteristic of nephrite derived from supersaturation of interacting fluids at low temperatures but acknowledge alternate possibilities.

Timing of Nephrite Development

Interpretation of the geologic history of the Granite Mountains and the timing of events related to nephrite development is based on investigations by numerous individuals. These include D.A. Bagdonas, K.R. Chamberlain, C.D. Frost, B.R. Frost, B.L. Fruchey, R.L.B. Grace, G.D. Langstaff, and E.N. Wall, noted in the references at the bottom of the page.

Most primary nephrite deposits in Wyoming appear to be related to Archean structures consistent with continental collision and accretion in a zone referred to by some as the Oregon Trail structural belt. The Oregon Trail structural belt extends eastward from the Wind River Mountains to the northern Laramie Mountains. This structure documents where Archean rocks to the south were accreted onto the Wyoming Craton to the north approximately 2.65 to 2.63 Ga (Ga = billion years ago). Numerous pervasive, dominantly ductile, east- to northeast-trending shear zones with steep southerly dips cut Archean rocks in this region.

The Granite Mountain batholith intruded the area around 2.62 Ga and is considered by Bagdonas (2014) to be part of the much larger Wyoming Batholith that extends westward into the southern Wind River Mountains and eastward into the Laramie Mountains. East- to northeast-trending mafic dikes intruded rocks of the Granite Mountains during two intervals at approximately 2.4 to 2.6 Ga and 1.47 to 1.45 Ga.

The first radiometric dating of nephrite in the Granite Mountains was by Peterman and Hildreth (1978). They obtained a piece of massive nephrite from a 10-cm-wide vein, which yielded 39Ar/40Ar dates of 2,460 ± 45 and 2,560 ± 45 Ma that they averaged to give an age of 2,510 ± 35 Ma (Ma = million years ago). They concluded diabase veins and nephrite deposits were both emplaced shortly after the intrusion of the Lankin Dome Granite, which they dated at 2,550 ± 60 Ma. More recent radiometric dating has refined the age of the Lankin Dome Granite at 2.62 Ga and the more extensive Wyoming Batholith at approximately 2,625 Ma.

Nephrite formation in central Wyoming is interpreted to be coincident with epidote formation adjacent to the nephrite deposits. Kevin Chamberlain (personal commun., 2014) dated epidotes associated with alteration zones surrounding Wyoming nephrite deposits from three different locations. One sample indicated progressive epidote growth from 2,477 to 2,462 Ma, while another indicated progressive growth from 2,550 to 2,520 Ma. A third location with a broad alteration zone yielded an age of 2,475 ± 11 Ma. This suggests that there may have either been two extended periods of epidote growth ca. 2,550 to 2,520 Ma and 2,477 to 2,460 Ma, or a much longer period extending from 2,550 to 2,460 Ma. The 2,550–2,520 Ma period is coincident with the intrusion of late magnetite-bearing plutons in the region (Chamberlain, personal commun., 2014), although the dominant period of regional plutonism occurred at about 2,625 Ma. General relationships suggest that nephrite and epidote are both either coeval with or post-date plutonism.

Alteration Zones

Although typical alteration zones accompany many Wyoming nephrite deposits, numerous jade veins have been observed with little or no accompanying alteration zones. This suggests that the hydrous fluids responsible for the jade at such locations reacted with the surrounding rocks only slightly or not at all. Jade, differentiated from actinolite-tremolite by jade’s microcrystalline structure, must have crystallized relatively rapidly at such sites from fluids supersaturated with actinolite-tremolite. The overall character of the host fluid must also have changed to prevent alteration of the surrounding rock. This may have resulted from either a rapid decrease in temperature or pressure, or both.

In contrast, some areas with wide alteration zones typical of nephrite deposits are known to host only low-quality nephrite or macroscopic crystals of actinolite-tremolite. Such areas likely had relatively slow changes in temperature and pressure that allowed larger crystal growth and greater penetration of the surrounding rocks by fluids.

Some of the highest quality jade is found surrounding large clusters of well-formed quartz crystals. The form of the quartz is typical of radiating crystals deposited on the walls of voids from hydrous fluids; these crystals are interpreted to have such an origin. The large size of the quartz crystals and absence of chalcedony suggests relatively slow and uniform quartz crystal growth in at least a temporarily stable environment. Some specimens are noted where high-quality nephrite surrounds brecciated quartz crystals, suggestive of an unstable environment in which nephrite was rapidly crystallized.

As with other geologic phenomena, there are variations. High-quality jade may surround quartz crystals with no perceptible alteration of the quartz; perhaps very rapid jade precipitation occurred due to an almost catastrophic change in temperature and pressure. Some quartz-jade interfaces show shallow alteration of the quartz crystals; these may have had slightly longer contact with the nephrite-bearing fluid before its crystallization. Some jade of lesser quality shows an almost complete replacement of quartz (and occasionally other minerals) crystals, leaving “ghost crystals” within the jade.

Many pieces of detrital jade have been found with the well-formed impressions of quartz crystals. The quartz was presumably removed from the jade during weathering. Some highly weathered, in-place “should-have-been nephrite” deposits are marked only by chalky-white to gray-green, relatively soft actinolite-tremolite surrounding well-formed quartz crystals.

In consideration of the above observations, it is possible that fluids related to batholith emplacement interacted with amphiboles to produce hydrous fluids supersaturated with actinolite-tremolite. Fluid chemistry was probably variable with time, location, and incident rock types. The active continental collision zone of the Oregon Trail structural belt most likely provided tectonic fractures as conduits for fluid movement. Such conduits would open or close in concert with tectonic activity, allowing either rapid entry or slow infusion of various hydrous fluids.

Slow infusion of fluids into these conduits would allow deep reactions with surrounding rocks resulting in wide alteration zones and relatively slow growth of quartz or actinolite-tremolite crystals. If these slow growth deposits remained within a temperature-pressure environment similar to that in which they formed, alteration processes would continue, resulting in serpentine or other end products. However, rapid entry of fluids into such conduits would allow fluids to ascend to cooler temperatures and lower pressures, where rapid precipitation from supersaturated fluids would produce the random microcrystalline structures found in nephrite jade.

Continuous sporadic uplift associated with continental collision could provide the necessary conditions for both formation of nephrite and its preservation. The process of jade formation apparently repeated erratically over time. This repetition is demonstrated where one type of nephrite deposit is cut by later veins of a different character.

Detrital Jade Deposits

Erosion of the core of the Granite Mountains subsequent to their uplift in the earliest Eocene produced thick conglomerates, primarily to the south. These conglomerates held boulders and cobbles of nephrite jade that had been tumbled for many miles. That tumbling broke up and destroyed much of the less resistant rock types and low-quality jade. Later during the Pliocene, the Granite Mountains were down-dropped, resulting in an accompanying reversal of stream flows to the north and further destruction of weaker rocks. Chemical and mechanical weathering further reduced less durable rocks and left a surface with an enhanced concentration of resistant rock, such as jade. Erosion of the Laramie and Wind River mountains similarly resulted in detrital jade deposits, but of lesser extent.

High-quality nephrite was found in residual and alluvial deposits as rounded boulders and cobbles during the early years of jade hunting. Although 80 years of intense jade hunting has removed most of the easily observable detrital jade, occasional finds, mostly of small pieces, are still made. The Eocene Ice Point Conglomerate, southeast of Sweetwater Station, at one time hosted relatively common boulders of apple-green, pink, and black nephrite jade. The Eocene Crooks Gap Conglomerate hosted apple-green jade boulders, but these were not abundant. Jade pebbles and cobbles have also been found in the Eocene Wind River, Battle Springs, and Wagon Bed formations. Similarly, jade has been found in the Oligocene White River, Oligocene-Miocene Split Rock and the Miocene-Pliocene Moonstone formations. Quaternary terrace, pediment, and alluvial fan gravels, as well as alluvium along modern drainages have also hosted detrital jade.

Nephrite may 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. 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.

History of Wyoming Jade

Minor prehistoric jade artifacts have been reported from Wyoming but have not been verified by archeologists. (Former) Assistant State Archaeologist Danny Walker (personal commun., 2010) examined carved prehistoric objects of various types from Wyoming, but although green in color, all have been steatite or serpentine rather than nephrite jade. The well-known early Wyoming Jade hunter, Allan Branham, spent many years hunting in central Wyoming for both artifacts and jade, and never found any jade artifacts. He concluded that the knowledge and skill to make use of jade was not present in the native inhabitants of central Wyoming as it was in other parts of the world such as New Zealand and Central America. The only prehistoric use of Wyoming Jade appears to have been for weighing down the bottom edges of buffalo hide teepees.

Anecdotal Accounts of Jade in Wyoming

Some of the earliest accounts of jade in Wyoming are anecdotal and were reported by Lawrence J. Bergsten in 1964. Bergsten talked with an old-time resident of Lander, Bill Marion, who related to Bergsten that prior to 1900, he had been accompanied on a prospecting trip through some rugged country by a Scottish geologist who picked up a rock and declared it to be jade.

Bergsten (1964) also related information given to him by Harvey Samuelson, a lapidary in Salinas, California, with 60 years of experience. While working in Portland, Oregon, as a lapidary in 1908 and 1909, Samuelson purchased apple-green Wyoming Jade from cowboys who had traveled west to spend their winters. The cowboys sold it for whatever they could get. Since there was no demand for the jade in the United States, most of it went to England. Bergsten accounted Samuelson’s story as reliable. Years later, about 1958 or 1959, Bergsten talked with a woman at a mineral and gem show in Indio, California, who had purchased jade jewelry from The Old Curiosity House in London, England, in 1909; the proprietor had told her at the time that the jade came from Wyoming.

Robert Hill, Sr. stated in a 1979 article in Gems and Minerals magazine that the first jade strike in Wyoming was in 1913, but provided no supporting data. In consideration of Bergsten’s anecdotes, it is possible that the existence of jade in Wyoming was known to at least a small number of individuals in the early 1900s. The jade strike in 1913 reported by Hill may have been a delayed reiteration of jade brought to Oregon four or five years earlier. However, there are no known early written records to substantiate this.

James L. Kraft, in his 1947 book "Adventure in Jade," related two stories concerning the discovery of Wyoming Jade that he had heard from local jade hunters. Kraft, the president of the Kraft Foods Company of Chicago, Illinois, had a great interest in jade, was one of the early purchasers of Wyoming Jade in about 1938 or 1939, and later financially helped Allan Branham open one of the first jade shops in Lander. Kraft’s jade hunting and personal contacts in Lander gave him the opportunity to hear the local stories concerning the discovery of jade in Wyoming.

One of these stories relates that a man named Corbin came to the area from Oregon on a bicycle to collect agates in the Sweetwater area (Sweetwater agate) in the early 1930s. During his search along the riverbanks, he picked up a chip of dark-green rock that he identified as jade. When cut and polished, it was exceptionally clear, and he showed it to a local gem collector, Biford Foster, so that Foster would know what to look for and maybe find more. Foster was a Lander lapidary, and he was considered a master craftsman in fashioning jade into jewelry. Corbin left the area for Oregon and did not return.

The second story relates that a sheepherder, working in the Crooks Mountain area in 1931, brought a sizeable piece of dark-green stone with quartz crystals to Lander that he had found. The stone turned out to be nephrite jade, which after identification, he sold to a museum. The story stirred up local interest and inspired local people to hunt for jade. This story was retold with less detail by Branham in 1941 and again with details by Russell P. MacFall as part of the introduction to his self-published collection of articles, "Wyoming Jade: A pioneer Hunter’s Story." That collection (publication year unknown) is dominated by articles written by Allan Branham that were reprinted from articles published in the Lapidary Journal in 1965 and 1966.

Bert Rhoads, an early and persistent jade hunter, found his first piece of Wyoming Jade in 1936. A Denver Post article, dated Feb. 1, 1948, credits Rhoads and his family with making known the gem potential of Wyoming Jade. On May 15, 1936, William L. Marion, a Lander hardware dealer, and Lloyd B. Curtis, a Lander science teacher, found both olive and intense-green and black jade in an outcrop north of Jeffrey City. However, Allan Branham later credits the Abernathy brothers with finding and mining the first primary, or in-place, deposit of Wyoming Jade.

A 1945 unfinished draft report on jade deposits in central Wyoming by U.S. Geological Survey geologist J. David Love stated that jade was discovered in the area about 1935, with unverifiable credit claimed by at least a dozen individuals. That statement probably reflects popular perception in 1945. However, Love correctly concluded that it will probably remain a mystery who or when the discovery of Wyoming Jade was made.

The Wyoming Jade Rush

The rush for Wyoming jade actually began in the late 1930s on a local level, but suffered an early setback due to WWII rationing of gasoline and a lack of automobile tires in the early 1940s. This kept most jade hunting local rather than hosting a large influx of people from other areas. However, many local people just couldn’t imagine that those green stones they had seen for years were worth picking up. Their attitudes changed when James L. Kraft, the wealthy cheese manufacturer and jade collector, visited Lander and purchased jade.

The early 1940s were busy times in the jade hunting areas of central Wyoming. Alan Branham in 1944 noted that when a large piece of jade was found, someone had to stay in the field to guard it until their partner returned from retrieving a wrecking car in Lander. Since no one had a large enough saw to cut it, the piece generally ended up staying in the backyard of the founder. Large jade boulder finds filtered down through the local grapevine and generally precipitated another influx of jade hunters to the jade fields.

An article titled “Green Gold of Wyoming” by Gold V. Sanders appeared in the February 1945 issue of Popular Science magazine and aroused interest across the country. Sanders’ article talked about “Fortunes lying loose on the ground,” and many citizens giving up their jobs to hunt jade, ... “Some have already become fairly wealthy.” The easy pickings, easy fortune tenor of the article published at the close of WWII set off the rush for Wyoming Jade.

The jade rush was similar to that of any gold rush. Some jade hunters were successful in making a small fortune in the jade fields and many found jade. Most jade hunters worked honestly, but those of a more unscrupulous character chose other methods to enrich themselves. Claim jumping, conflicts, legal challenges, and a few nighttime holdups led to secrecy concerning jade hunting areas. However, secret hunt areas rarely remained secret for long, and the easy pickings of detrital jade slowly dried up during the 1950s and 1960s.

After the Rush

Interest in Wyoming Jade never really ended; the rush slowly wound down and was gradually supplanted by more earnest work-related efforts to make a reasonable livelihood hunting for or mining jade. Jade mines, focusing on in-place deposits in the central Granite Mountains, began as early as 1936 and operated into the 1980s. Production from these mines began slowly, peaking in the late 1970s. Wyoming newspapers in the 1960s and 1970s occasionally focused articles on various jade businesses, touting their economics. Rock collecting and related clubs were also active during this period. It appears that both amateur and professional interests contributed to the designation of Wyoming Jade as the official gemstone of the State of Wyoming on Jan. 25, 1967.

Jade excitement tapered down during the 1980s, after which only small amounts of jade were mined on an erratic basis. Interest in Wyoming jade slowly revived, beginning about 2005. This partially reflected an increased interest in jade carving in China, California, and in other parts of the world. As of 2018, at least two sites have been the focus for small-scale jade mining operations, and numerous jade claims have been staked or re-staked in central Wyoming. A review of literature related to Wyoming Jade suggests that more jade has been derived from detrital sources over the years than from in-place deposits. However, the total amount of jade produced in Wyoming is unknown, as is actual production from either mines or from surficial deposits.

Wyoming Diamonds

Diamonds were discovered in southeastern Wyoming in 1975 about 25 miles south of Laramie. Since then, more than 130,000 diamonds have been found in the Colorado-Wyoming State Line kimberlite district. These vary from microdiamonds to high-quality gems, with one as large as a 28.3-carat diamond found in Colorado. The largest diamond found in Wyoming to date weighed 6.2 carats.

Diamond, composed of pure carbon, is the hardest naturally occurring mineral found on Earth. Diamonds form under extreme pressure and high temperature deep within the earth’s mantle. They arrive at the surface through volcanic processes that carry them upward in rare magmas (melted rocks) known as kimberlites and lamproites. A few other rock types may transport diamonds from the mantle, but have not yet demonstrated commercial diamond production. Both kimberlites and lamproites occur in Wyoming, as do some less well-known potential diamond host-rocks.

Most of Wyoming has high potential for the occurrence of diamonds. The ancient core of the North American continent, the Archean craton, extends southward from Canada and lies beneath most of Wyoming where it is known as the Wyoming craton or Wyoming Province. This stable part of the continent is more than 2.5 billion years old (2.5 Ga) and is believed to have a high potential for diamond deposits. Slightly younger 1.6 to 2.5 Ga accreted parts of the continent have a moderate potential for diamonds. However, the accreted terrain of the Proterozoic Colorado Province includes numerous diamond-bearing kimberlites located in Colorado and along the Wyoming-Colorado border in the State Line kimberlite district.

Although most of Wyoming has not been explored for diamonds, several diamondiferous kimberlite and related host rocks have been discovered in the state, along with some unrelated placer diamonds. The sources of these placer diamonds remain unknown.

Diamond-bearing rocks crop out in the Wyoming-Colorado State Line kimberlite district, in the Iron Mountain District in the east-central part of the Laramie Mountains, and in the Cedar Mountain area southwest of Green River. The diamond-bearing rocks outside of the State Line district have not been fully evaluated, and some have not been completely mapped.

Exploration

Diamond exploration often begins with a sampling program to locate indicator minerals, such as pyrope garnets and chromian diopside.

Concentrations of indicator minerals point to the potential presence of nearby hidden kimberlites and diamond deposits. During the last 30 years, the WSGS has identified several hundred concentrations of kimberlite indicator minerals, indicative of possible nearby hidden diamond deposits. Further exploration may involve geophysical surveys and drilling to identify and determine the size of kimberlites or other potential diamond-hosting rocks.

Because kimberlites and related diamond host rocks tend to be deeply weathered, they often occupy areas of low relief or are covered by deep soil and debris from adjacent rocks. This makes them quite difficult to find in most areas, although local conditions may allow some to stand out in relief. Soils derived from weathered kimberlite contain abundant montmorillonite clay, often support more vigorous growth of grasses than do surrounding areas, and may show a marked absence of woody plants. These vegetative anomalies and topographic depressions are clues that may point to the existence of hidden kimberlitic intrusions. Summaries of kimberlites and diamond exploration in Wyoming can be found in WSGS Reports of Investigations 53 and 54.

Mine Development

Development of a diamond mine begins with extensive exploration followed by claim staking where the minerals are federally owned or by leasing of state- or privately-owned minerals. However, finding economic diamond deposits is much more difficult than locating deposits of other minerals. Even in a world-class diamond mine, diamonds account for less than one part per million in the host rock. Once a diamond-bearing rock has been located, sampling to evaluate the diamond content of the deposit begins. Sampling progresses in stages beginning with a few tons. With favorable results, sampling increases to a few hundred tons and then to a few thousand tons. Continued favorable diamond showings at each step will eventually lead to full-scale mining.

A small Colorado diamond mine operated along the Wyoming-Colorado border from 1996 to 2003. The ore grade varied from 5 to 15 carats per 100 tons, and the mine produced many high-quality gems larger than one carat in size. The largest diamond extracted there weighed 23.8 carats. However, the mine closed due to legal problems rather than a lack of diamonds. The State Line kimberlite district has never been fully evaluated.

Economics

Diamonds are valued primarily as gemstones. Uncut diamond prices climbed nearly 50 percent between 2002 and 2006 due to growing worldwide markets. Prices retreated substantially following the 2008 recession, but recovered by 2011. Since 2012, diamond prices have fluctuated in response to various factors in the world economy and emerging competition from synthetic diamonds, according to Spektorov and others (2012). Recently, several prospectors, and at least one company, were actively exploring diverse locations within Wyoming for kimberlites and diamonds during 2017 and 2018.

The four Cs of color, clarity, carat weight, and cut determine the value of individual diamonds. A slight increase in carat weight can dramatically increase the value of a diamond. Exceptional stones command much higher than average prices, and cutting rough stones may increase their value by 10 times or more.

Diamond mines are based on the presence of gem-quality diamonds (larger is always better). The smallest size of recoverable diamonds, when defining ore reserves, is specific to each mine and mill. A mine’s reserves represent the economic material around which it is designed. Some large mines in Canada (the world’s third largest diamond producer) include diamonds as small as 2 to 3 millimeters in their ore reserve calculations. Low-quality and extremely small diamonds are used as abrasives but are not profitable to mine in the absence of gems.

The potential for new diamond discoveries in Wyoming is great, as is the possibility for one or more diamond mines in Wyoming’s future. Untapped prospecting opportunities for placer diamonds downstream from known kimberlites, and in areas where placer diamonds have been reported in the past, also abound in Wyoming. WSGS Information Pamphlet 12, Searching for Placer Diamonds by W. Dan Hausel (2004) gives detailed information for prospectors interested in hunting for placer diamonds.