One Active Mine Today ...
The United States only has two locations that have been operated as commercial diamond mines. One was the
Kelsey Lake Diamond Mine near Fort Collins, Colorado. It produced small amounts of diamonds between 1996
and 2002 when the mine was closed due to legal problems.
The second is at Crater of Diamonds State Park near Murfreesboro, Arkansas. It was worked as a commercial
diamond mine in the early 1900s but closed because the deposit was subeconomic. It has since been operated
by the State of Arkansas as a tourist "pay-to-dig mine" where anyone can pay a fee, look for diamonds and keep
any that they find. A few hundred carats of diamonds are found there each year.
Diamonds, diamond indicator mineral assemblages and potentially diamondiferous rocks have been found at hundreds of locations within the United States. One or more of these locations could become commercial and make the United States a significant producer of gem-quality diamonds. 
Crater of Diamonds - The Only Active U. S. Mine
Although millions of carats of diamonds are consumed each year in the United States, only a few hundred carats are
domestically produced. The only active diamond mine in the country is the
Crater of Diamonds Mine near Murfreesboro, Pike County, Arkansas.
There, recreational prospectors have been finding a few hundred carats of diamonds per year since the early 1970s.
Most of the stones are white, yellow and brown in color but a wide range of diamond colors has been found at the mine. See photo at top right.
>> The only diamond mine where you can be the miner. <<
The mine is a dig-for-fee operation maintained by the Crater of Diamonds State Park.
The diamonds are hosted in a lamproite breccia tuff and its overlying soil in a structure known as a maar. Collectors pay a fee of a few dollars per day to
prospect and can keep any diamonds that they find. This is the only diamond mine in the world that is open to the public.
Crater of Diamonds has produced several significant finds. These include:
The "Strawn-Wagner Diamond"
This diamond was found at the Park as a 3.03
carat rough stone in 1990 by Shirley Strawn. In 1997 it was cut to yield a 1.09 carat, "round brilliant" stone that received a perfect grading of 0/0/0 from
the American Gem Society. It stands as the most perfect diamond the American Gem Society has ever certified. 
The "Uncle Sam" Diamond
The "Uncle Sam" is a 40.23 carat white diamond that stands as the largest diamond ever found in North America. It was found at the Crater of Diamonds in 1924 before the property was opened as a State park. 
The Kahn Canary Diamond
This 4.25-carat, canary-color diamond with a perfect dodecahedral pillow shape was found in 1977 by George Stepp. Stan Kahn purchased the diamond from Stepp and did not have it cut because even in its rough shape it is an especially beautiful gem. Kahn has shared the stone with the public by loaning it to museums around the world for temporary display.
Kelsey Lake Mine
At present, there are no commercial diamond mines operating in the United States.
The only commercial mine with recent activity is the Kelsey Lake Diamond Mine, located near Fort Collins, Colorado on
the state boundary between Colorado and Wyoming.
Kelsey Lake was opened as a
commercial diamond mine by Redaurum Limited in 1996. Great Western
Diamond Company, a wholly-owned subsidiary of McKenzie Bay International
Limited, purchased the property in 2000 and operated the mine until 2002. It was closed
due to legal problems rather than a lack of diamonds. 
Most of the diamonds produced at the Kelsey Lake Mine were clear,
gem-quality stones. About one-third of the stones were one carat or
larger in size. When the mine closed there was an identified resource
of 17 million tons of ore with an average grade of 4 carats per hundred metric tons. In 2003 the United States Geological Survey reported that the most valuable stones produced from the Kelsey Lake Mine were valued at $89,000 and $300,000. 
Wyoming Diamond Deposits
The Wyoming Geological Survey believes that a billion-dollar diamond industry
could develop in their state. Wyoming has comparable conditions to the Canadian deposits and
hundreds of kimberlite pipes are believed to exist. Their website has maps showing the location
of diamond-hosting intrusives, reported placer diamonds, kimberlite indicator mineral anomalies,
lamproites and diamond stability indicator minerals. 
Some points that communicate their optimism for a significant diamond industry in Wyoming :
>> Diamond Mines in Canada <<
- 40 diamond deposits in the State Line District
- 130,000 diamonds recovered from the State Line District
- several diamonds weighing more than 28 carats
- hundreds of kimberlitic indicator mineral anomalies
- Wyoming has the two largest kimberlite districts in the U.S.
- Wyoming has the largest lamproite field in North America
Other Potential Diamond Areas
The discovery of numerous commercial diamond deposits in Canada has generated
prospecting interest in the United States. Areas with similar geologic settings to the
Canadian deposits exist in Alaska, Colorado, Minnesota, Montana, and Wyoming. Diamond
indicators and confirmed diamond pipes have been found but so far none have attracted significant
investments or commercial mining.
Hundreds of diamonds have been found in the Pacific coast region; however, that area is unfavorable
for the presence of kimberlite and lamproite. It is possible that these diamonds are coming from
a source that has yet to be understood. 
Finally, several companies in the United States are now producing synthetic diamond using
chemical vapor deposition and other processes. The diamond produced by these companies can be
used as gemstones or as high-performance materials in manufacturing processes. Synthetic diamonds
have a large number of uses and have replaced natural diamonds in most industrial processes.
Contributor: Hobart King
Find it on Geology.com
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| Photograph of several diamonds found at the Crater of Diamonds State Park, near Murfreesboro, Arkansas. These diamonds were probably nicely formed crystals when they were in the Earth's mantle. Their shapes were modified by corrosive fluids during their rapid ascent to Earth's surface. Photo used with permission of Crater of Diamonds State Park.
| A small portion of a diamond exploration map published by the Wyoming Geological Survey. WSGS has identified several hundred concentrations of kimberlite indicator minerals, indicative of possible nearby hidden diamond deposits. Image by Wyoming Geological Survey. 
|Simplified cross-section of a lamproite pipe and residual soil deposit similar to the one at the Crater of Diamonds Mine.
|Photograph of the famous "Strawn-Wagner Diamond" found at Crater of Diamonds State Park in 1990 by Shirley Strawn. It was the first stone to receive a perfect grade of 0/0/0 from the American Gem Society. Image courtesy of Crater of Diamonds State Park.
|Another type of "diamond" mined in the United States is the "Herkimer Diamond" of western New York. These are not genuine diamonds - instead they are doubly-terminated quartz crystals that form in vugs of the Little Falls Dolostone. Many people "mine" for them at a few fee mining sites. They have been known since Native Americans found them in the streams of the area. Today they are considered a mineralogical novelty because of their natural doubly-terminated shape. The fee mining sites are very popular and attract thousands of visitors from around the world each year.
 Hausel, W.D. (1998). Diamonds and Mantle Source Rocks in the Wyoming Craton with a Discussion of Other U.S. Occurrences. Report of Investigations Number 53, Wyoming Geological Survey.
 Arkansas Department of Parks and Tourism (accessed May, 2013). Crater of Diamonds State Park website.
 Olson, Donald W., (2003). 2003 Minerals Yearbook: Gemstones. 2003 Minerals Yearbook, Volume I, Metals and Minerals, Gemstones. United States Geological Survey.
 Wyoming Geological Survey. Wyoming Diamonds. Wyoming Geological Survey Website (accessed May, 2013).
 Wyoming Geological Survey. Diamond Exploration and Mining in North America Heating Up. Wyoming Geological Survey Website (accessed May, 2013).
 Howard, J.M. and Hanson, W.D. (2008). Geology of the Crater of Diamonds State Park and Vicinity, Pike County, Arkansas, State Park Series 03, Arkansas Geological Survey.
 Olson, Donald W., (2011). 2009 Minerals Yearbook: Gemstones. 2009 Minerals Yearbook, Volume I, Metals and Minerals, Gemstones. United States Geological Survey.