Over the past 30 years, hundreds of previously unrecorded gem occurrences were identified in Wyoming & nearby regions including diamond, ruby, sapphire, pyrope garnet, Cape Ruby, Cape Emerald, chromian diopside, peridot, opal, kyanite, gold, agate, jasper, iolite & others by using geological science & geochemistry. Two deposits of iolite are world-class deposits & one opal deposit is one of the largest in the world.
I love searching for diamonds. My first experience in diamond hunting occurred a few decades back when I mapped the Wyoming State Line kimberlite district. I later mapped the Iron Mountain and Sheep Rock kimberlite districts and also the Leucite Hills lamproite field. In when I had time, I explored for diamonds for some US companies, an Australian company and some Canadian companies.
I searched for kimberlite, lamproite and lamprophrye in California, Kansas, Montana & Wyoming & identified several hundred cryptovolcanic structures within & surrounding the State Line in Colorado and Wyoming that are likely diamond deposits (these remain unexplored). A few include Indian Guide, Twin Mountain, Happy Jack & others. I expanded my research & found cyptovolcanic structures in Canada & even in the Kimberley region of South Africa. I found a major district of 50+ anomalies sitting along the interstate in the US!
The diamond deposits south of Laramie are in kimberlite & placers. The kimberlites are deeply eroded & spilled millions of diamonds into the surrounding streams, but no one ever systematically looked for diamond in the creeks (even so, diamonds were accidentally recovered in Rabbit Creek along with a 5 carat diamond, and hundreds were recovered in George Creek, and several including a 6.2 carat diamond were recovered in Fish Creek, but the rest of the streams are UNPROSPECTED!
Kimberlite is a ultrabasic, potassic igneous rock that erupts along fractures from 90 to 120 mi depths. They typically occur in very old cratons & cratonized rocks (basically ancient continental cores that consist of >1.5 billion year old granite, gneiss & schist). The magma, under pressure rises rapidly from the mantle because of the great depth & because of considerable water vapor & carbon dioxide under pressure. Some suggest gaseous emplacement velocities of kimberlite are on the order of Mach 3. The eruption is relatively cool: CO2 gas expands cooling the magma such that emplacement temperatures of 32 degree F are not uncommon. This collection of unusual characteristics results in small, circular maar-like volcanoes (without cones) & dikes that are structurally controlled.
Gem diamond with excellent characteristic trigons on surface
Things to keep in mind: kimberlite will serpentinize because of water vapor, this produces a relatively soft rock that erodes faster than surrounding country rocks & usually results in a depression with different vegetation than the surrounding rocks. These depressions may contain shallow ponds. They are structurally-controlled such that >one anomaly is often found in a line. Because of calcium carbonate in kimberlite, carbonate will leach out into the pond staining the soil white. Keep in mind that salts are not all that uncommon in basins where lots of young sedimentary rocks occur with considerable carbonate. But in the craton basement (i.e., mountain ranges of Wyoming) there is no known source for carbonate, so if you spot a structurally-controlled lake surrounded by salt in old Precambrian rock, you might want to find out why? And if you find one, typically, with effort, you will find others along the same structure.
Diamonds found in Colorado & Wyoming ranged from microdiamonds to 28.3 cts & included one chip from a 80 to 90 carat stone. Some assume there are no commercial diamond deposits in this region - but all of the mills that were constructed were poorly designed and rejected as many diamonds as they recovered. Even so, commercial diamond deposits were encountered at Kelsey Lake and mined for only about 6-months before lawsuits shut down the operation. Thus, only the very top of the diamond pipe was touched by mining and there are thousands (if not many millions) of gem-quality diamonds in the host kimberlite that remain unmined!
Then there is the Sloan 1 and 2 kimberlites in Colorado. DiamonEx from Australia was developing this property for commercial tests when the economy crashed in 2008 and put most diamond operations out of business including DiamonEx Ltd.
All of the mills in the State Line district were so poorly designed they rejected diamonds of all sizes. The Kelsey Lake mill rejected anything weighing >40 cts! It also rejected most diamonds under 40 cts such that when the tailings were tested in 1997, the first sample yielded a 6.2-ct gemstone along with many other diamonds! The grades of several kimberlites were high, the gem:industrial ratios were good & diamond values were reasonable. The biggest problem with the State Line district was good diamond companies with diamond expertise were in short supply.