Ref.: The American Museum of Natural History.
The "State Line Kimberlite District" along the Colorado-Wyoming border is home to the only operating diamond mine in the U.S., started at Kelsey Lake, Colorado, in June 1996. The largest diamond found so far is 28.3 carats, and about 25% of the 20,000-carat annual production is of gem quality.
There are eight kimberlite pipes known in the mine area. The 1 and 2 pipes were open pit mined (about 20 acres of surface exposure).
