During several brief episodes of exploration, >130,000 diamonds were recovered from the State Line district, Colorado-Wyoming (5). A few diamonds were recovered from at least one kimberlite in the Iron Mountain district, Wyoming (6), several diamonds were recovered from a lamprophyre complex at Cedar Mountain, Wyoming, a diamond was recovered from the Homestead Kimberlite in Montana (7), a diamond was reportedly recovered from the Winkler kimberlite in Kansas (8), and several placer diamonds were reported in Wyoming, Montana, Colorado, Idaho, California, Oregon and Washington (9).
A 14.2 carat octahedron recovered at Kelsey Lake, Colorado.
In the State Line district, four differnt mills were constructed. All were poorly designed such that contamination and security was a problem, and all were notorious for rejecting diamonds of all sizes. It is likely that as many diamonds reported to the tailings as was recovered (and in some cases, probably more) and large diamonds were likely rejected at all of the mills. The Kelsey Lake mill in particularly was documented to have been designed to reject everything larger than 40 carats. After the recovery of some large diamonds, three weighing >15 carats (two >28 carats) and a fragment from a much larger stone, it was realized the company had made a grave error in cutting costs. Even so, the mill was also known to reject many diamonds smaller than 40 carats. After the mine had terminated operations, the first sample of tailings tested yielded several diamonds including one >6 carats!
About 50% of the diamonds recovered from the Wyoming kimberlites were high-quality gemstones. About 30% from the Kelsey Lake were gem-quality. Diamonds included transparent white, grey, black, brown, yellow and there were even reports of some with pinkish tone!
Based on all of the known kimberlites and all of the known anomalies, this is one of the greatest diamond targets in the world (especially since it is located adjacent to a US Highway and an Interstate system.
Left and below - Parcel of gem-diamonds from Schaffer and Aultman kimberlites, Wyoming.
Colorado diamonds including rough from Kelsey Lake (left) and a faceted 16.8 carat stone (right) produced from a 28.18 carat rough.