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Historically interesting discoveries of large diamonds in the eastern United States include the 23.75-carat Dewey Diamond found in Manchester, Henrico County, Virginia, in 1885 and the 34.46-carat Punch Jones Diamond found by horseshoe players in Peterstown, Monroe County, West Virginia, in 1928. Reported discoveries in gold placers include those at the Vaucluse mine, Orange County, Virginia; small stones from Brindletown Creek, Burke County, the J. D. Twitty placer in Rutherford County, mines in Mecklenburg County, the Portis mine, Franklin County, and a placer in McDowell County, North Carolina; and mines in Hall and White Counties, Georgia. Other well-documented finds include a 4.33-carat stone found on a farm at Dysortsville, North Carolina; a 4.5-carat stone found at Morrow Station, Clayton County, Georgia; and stones of 3.5, 2.41, and 4.25 carats respectively from Lee, St. Claire, and Shelby Counties, Alabama. Additional poorly documented discoveries are reported from Tennessee and South Carolina.
Diamonds in the upper Midwest have been reported sporadically since 1876 when a pale yellow dodecahedral crystal weighing 15.37 carats was found in the Kettle Moraine near Eagle, Waukesha County, Wisconsin. Other Wisconsin diamonds include a 21.25-carat stone found near Kohlsville, Wisconsin, in 1886 (it was cut in 1918 yielding stones totaling 9.27 carats); several small crystals from Plum Creek, Pierce County; a 3.87-carat stone from near Oregon, Dane County; and a flattened trisoctahedron of approximately 6.5 carats from near Saukville, Ozaukee County. Other important finds include a 10.88-carat stone from Dowagiac, Cass County, Michigan; a 7.75-carat crystal from near Ashley, Jefferson County, Illinois; and several stones found by panning in Gold Creek and nearby creeks in Morgan and Brown Counties, Indiana. Unsuccessful exploration of kimberlite has taken place in Elliot County, Kentucky. More recently, modern exploration has resulted in the reported identification of uneconomic though diamondiferous kimberlites in Iron and Dickinson Counties in northern Michigan.
The most famous diamond occurrences in the United States are those near Murfreesboro, Pike County, Arkansas (Kidwell 1990). The first diamonds were found at the most productive of these, the Prairie Creek intrusive, by John Wesley Huddleston in 1906, although the presence of kimberlite-like rock had been recognized in Pike County as early as 1889. Attempts to mine the property were made off and on for decades, with one period of testing in 1912 resulting in the recovery of more than 100,000 diamonds. The average grade of the deposit was reported to be only about 0.1 carat per ton with an average stone size of 0.26 carat. The deposit was operated as a tourist attraction between 1949 and 1972, at which time it was purchased by the Arkansas Department of Parks and Tourism. It has been operated as the Crater of Diamonds State Park since that time. Promotional literature indicates that 12,958 diamonds weighing a total of 2,784.82 carats were found by tourists between 1972 and 1988, an average of 145 visitors per diamond. Large diamonds from the deposit include the Uncle Sam, 40.25 carats; the Star of Murfreesboro, 34.25 carats; the Amarillo Starlight, 17.86 carats; and the Star of Arkansas, 15.33 carats. The deposit was evaluated as a viable diamond deposit by a consortium of companies during the past decade and found wanting. Interestingly, detailed study of the host "kimberlite" during this work resulted in its reidentification as lamproite. Modern evaluation of other nearby occurrences resulted in the same generally uneconomic conclusions, although several relatively large stones were reportedly found in the process. In recent years, geophysical surveys over the adjacent Coastal Plain led to the identification of anomalies consistent with shallowly buried ultramafic intrusives; confirmatory drilling resulted in the discovery of at least one previously unknown "kimberlite" pipe. In 1926 a perfect 27.21-carat octahedral diamond crystal was found by a young girl picking cotton near Searcy, some 140 miles from the Murfreesboro occurrences.
Numerous, generally small diamonds were found in the gold placer operations in Amador, Butte, and El Dorado Counties, California. More than three hundred stones are attributed to Cherokee Flat alone and lesser numbers to Thompson's Flat and Yankee Hill in Butte County. El Dorado County finds include those at Forest Hill, Webber Hill, White Rock Canyon, Cedar Ravine, and Smith's Flat. Other localities include French Coral in Nevada County, where a 7.25-carat stone was found, and placers in Del Norte and Humboldt Counties. Four nongem diamonds of 3.90, 14.33, 17.83, and 32.99 carats were found by a retired geologist in the 1980s on a tributary of the South Fork of Trinity River, Trinity County (Kopf, Hurlbut, and Koivula 1990).
In 1964 odd "breccias" along the Colorado-Wyoming state line were recognized as kimberlites. In 1975 diamond was accidentally discovered in a sample from the Shaffer diatreme in what is now known as the State Line district (Collins 1982), resulting in a period of vigorous exploration during which the Sloan, Schaffer, Aultman, and Kelsley Lake kimberlites were evaluated by mining companies. The Kelsey Lake deposit in Larimer County, Colorado, was operated on a trial mining basis between 1996 and 1999. Well-developed octahedra weighing up to 14.2 carats were recovered; other stones of 28.2 carats (cut into a 16.8-carat finished gem) and 16.3 carats were recovered; the largest stone found was a 28.3-carat yellow diamond of somewhat irregular shape. Good Kelsey Lake diamonds (fig. 5) have been for sale in recent years at the Tucson Gem and Mineral Show.
[Figure 5 ILLUSTRATION OMITTED]
Other isolated, generally accidental American diamond finds include an exceptional gem crystal of 18.2 carats from a housing development in Princeton, Bosier County, Louisiana; a 4.5-carat stone found in Quay County, New Mexico, in 1909; a gem-quality rounded octahedron from near Craig, Lewis and Clark County, Montana, that was found in 1990; a single stone from Walker County, Texas; and several stones recovered during placer mining at Crooked Creek, Circle district, Alaska.
Development of a major diamond mining district in Canada's Northwest Territories is currently taking place, with the first, the Ekati deposit, having been recently brought on stream. The pipelike bodies, the discovery of which sparked a major staking rush, are north of Lac de Gras in a very remote area 300 kilometers northeast of Yellowknife. Although specimens have just begun to appear on the collector market, exceptional gem octahedra were reportedly recovered during bulk testing at the Ekati site. Aggressive exploration has confirmed the presence of diamond-bearing kimberlite pipes scattered over a considerable area extending into adjoining Alberta, Saskatchewan, and Nunavut.
Diamonds have been known in South America at least since the 1720s, the time of their discovery in Brazil. At first they were mere curiosities related to gold placer operations, but Brazil quickly displaced India as the leading world source, a position it held until the development of the South African mines in the 1870s. A good historical treatment of Brazilian diamond occurrences as they were known through the nineteenth century was given by Bauer (1904) and an updated review of the fourteen major districts by Cassedanne (1989). Most production has been from the "Triangulo Mineiro" area of Minas Gerais (particularly the Diamantina district), Mato Grosso, Roraima, and Bahia, with lesser though still significant output from the states of Parana, Goias, Sao Paulo, Maranhao, Paro, and Piaui. Rivers with significant historical diamond production attributed to them include the Doce, Corrego Coro, Rio das Velhas, Jequitinhonha, Pardo, Tibagy, and the Rio das Garcas. Significantly large stones have been found in the Abaete River at Tiros and the Santo Antonio River, Minas Gerais, and in the Lancois district, Bahia (carbonado). At least thirty-six diamonds in excess of 100 carats have been discovered in Brazil, including the 108-carat Benito Valadares Diamond and a 350-carat black diamond from Bahia; the pale blue 176-carat Brazilia Diamond; the 194-carat lilac-colored Cedro do Abaete Diamond; the 600-carat Goiaz Diamond; the 227-carat Mato Grosso Diamond; the 324-carat Patos Diamond; the Presidente Dutra (409 carats) and the Presidente Vargas (727 carats); the Darcy Vargas (455 carats); and finally the 328-carat Vitoria Diamond. Large Brazilian carbonados include the Carbonado Casco de Barro (about 2,000 carats), Carbonado do Sergio (3,167 carats), and Carbonado Xique-Xique (932 carats). Small though well-formed diamond crystals have been available in recent years from the Diamantina district, Minas Gerais.
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