Wyoming is a geologically rich region, offering a diverse array of mineral and gemstone finds for collectors. This abundance results from the state’s ancient and varied geologic history, including mountain-building periods and significant volcanic activity. The deeply rooted Archean craton underlies much of the state, providing the necessary high-pressure, high-temperature environments for rare gem formation. Sedimentary rock layers and younger volcanic deposits further contribute to a wide variety of collectible materials.
Wyoming’s Signature Gem: Nephrite Jade
Wyoming’s most famous geological treasure is nephrite jade, officially designated as the state stone in 1967. This calcium magnesium silicate is known for its exceptional toughness and durability, making it highly valued by carvers. Wyoming jade quality is often used as a benchmark for comparison with nephrite found elsewhere.
The most common colors are various shades of green, such as olive, leaf, and apple green, but Wyoming nephrite also occurs in rarer colors ranging from white to black. The highest quality pieces are distinguished by their lighter, brighter colors and a desirable degree of translucency. However, only about ten percent of nephrite deposits are considered gem quality.
Historically, the highest quality nephrite has been found as rounded boulders and cobbles in alluvial deposits throughout central Wyoming. This area extends from the southern Wind River Mountains, across the Granite Mountains, and eastward toward Guernsey. The region around Jeffrey City and the Granite Mountains is considered the center of the state’s jade fields, but easily found surface material has been largely depleted.
Common Collectible Minerals: Agates, Jaspers, and Petrified Wood
Moving beyond jade, Wyoming offers a wide range of accessible, silica-based minerals, which are highly popular among casual collectors. These include numerous varieties of agates and jaspers, which are both forms of chalcedony, a microcrystalline quartz. Some of the more notable agates found in the state are dendritic, moss, and fortification agates, often collected as river-worn cobbles.
A particularly unique find is the Sweetwater Agate, which occurs as small pebbles in lag gravels, especially near the Split Rock Formation. These agates are known for their striking fluorescence, exhibiting a bright green glow under shortwave ultraviolet light. This unusual trait is caused by trace amounts of uranium present as the uranyl ion within the mineral structure.
Various forms of jasper are also abundant across the state, including a brecciated variety known as Youngite, found near Guernsey. Petrified wood, another form of silicified material, is widely available and often found in hues ranging from black to opalized and agatized forms. Collecting petrified wood is generally permitted on public lands, but it is subject to special rules, including a daily limit of 25 pounds plus one piece, not to exceed 250 pounds per year for personal, non-commercial use.
The Geology of Diamond Finds
Wyoming’s geological context provides a rare setting for the presence of diamonds, primarily in the southeastern portion of the state. These diamonds formed deep within the Earth’s mantle, requiring immense pressure and high temperatures, and were brought swiftly to the surface through volcanic events. The host rock for these diamonds is typically kimberlite, or sometimes lamproite, which are rare magmas that intrude into the crust.
The most significant area for diamond discovery is the Colorado-Wyoming State Line district, located south of Laramie, which is part of the Archean Wyoming Craton. This region contains numerous kimberlite pipes and has yielded over 130,000 diamonds, ranging from microdiamonds to high-quality gems. The largest diamond recovered in Wyoming weighed 6.2 carats, though high-quality faceted stones have been found.
While many of the recovered diamonds are industrial grade, the presence of gem-quality stones confirms the district’s potential. The State Line district and the Iron Mountain district to the north represent two of the largest kimberlite districts in the United States. These deposits tap into the ancient cratonic basement, suggesting a high potential for undiscovered diamond deposits.
Public Rockhounding and Collection Resources
For those interested in collecting, the Bureau of Land Management (BLM) manages nearly 18 million acres of public land in Wyoming that are generally open for rockhounding. The public is typically allowed to collect reasonable amounts of rock material and nonrenewable resources for non-commercial purposes without charge. Collection should be done using only hand tools, and any surface disturbance must be reclaimed after finding specimens.
It is important to understand the distinctions between public land jurisdictions, as National Parks and State Parks often prohibit the removal of geological features. Collecting on private land always requires explicit permission from the landowner. Collectors should also be aware of mining claims in certain areas, as collecting within an active claim can violate the rights of the claimant.
Safety considerations are paramount when exploring the state’s remote areas, including watching for wildlife and carrying essential gear. Recommended tools for any expedition include a rock hammer, safety glasses, and a reliable navigation device. The best time for rockhounding in Wyoming is generally during the summer and early fall when winter conditions do not limit access.