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.

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.