[1] Scientists agree the diamond zones vary from about 90 to 125 miles below the surface, where a certain combination of heat and pressure allows carbon to crystallize into gems.  With few exceptions, they form below the stable, central mass of continents-technically the “cratons.”

    At the beginning of the 20th Century, many geologists still thought diamonds crystallized after magma picked up carbon while pushing through shale near the surface:  the carbon supposedly crystallized after the volcanic deposit settled.   That theory lost credibility, however, as the walls of various deep mines in South Africa showed no evidence of shale deposits.  For concise comment on this topic at the time, see Philip F. Schneider, “A Unique Collection of Peridotite,” Science, 28 (July 17, 1908), 92-93.  That statement echoed the ideas of Sir William Crookes, an English diamond expert who theorized that “at a sufficient depth there were masses of molten iron at great pressure and high temperature, holding carbon in solution ready to crystallize out of cooling [of the earth’s crust]” (for a summary of his ideas, see “Origin of Diamonds Still a Mystery,” New York Times, September 21, 1930, Sec. 9, p. 11).

    In 1914, Hugh D. Miser of the U.S. Geological Survey (USGS) concluded that the peridotite intrusions in Pike County occurred “later than deposition of the Trinity formation, which is Lower Cretaceous, and earlier than that of the Bingen sand, which is basal Upper Cretaceous.  As the Upper and Lower Cretaceous rocks in this region are separated by an unconformity representing an uplift sufficient to raise the region about sea level at the close of the Lower Cretaceous, it seems reasonable to assume that the intrusion of the peridotite (the basic magma) and possibly other igneous rocks of the State accompanied the diastrophic movements producing this elevation” (Miser, New Areas of Diamond-Bearing Peridotite in Arkansas, USGS Bulletin 540 [1914], 545;  later restated in various editions of the final USGS survey, 1922-1929).

    Numerous studies of the Pike County formation have been published since 1907.  The one most often cited is Hugh D. Miser and A. H. Purdue, Geology of the DeQueen and Caddo Gap Quadrangles, Arkansas, U. S. Geological Survey Bulletin 808 (1929).  That edition restates the point Miser made in previous reports:  “The three types of peridotite were apparently formed by three distinct volcanic outbursts, but they are so closely related that they probably represent successive stages in a single period of volcanic activity” (114).  Recently, geologists reclassified the Crater of Diamonds and added a fourth, minor category, “epiclastic rocks,” found at points on the periphery.  The label indicates rock made primarily of material of volcanic origin.

    For perspective, including theories of origins and pipe formation, see “The Nature of Diamonds,” American Museum of Natural History:  http://www.amnh.org/exhibitions/diamonds/.  An excellent bibliography is included.

[2] Leading scientists began accepting the “violent eruption” concept at the turn of the century.  Again, Sir William Crookes was a pioneering theorist (“Origin of Diamonds Still a Mystery,” ibid.). Miser and Ross, “Diamond-Bearing Peridotite,” 672, echoed Crookes’ central point:  “In Arkansas as in South Africa the diamonds come largely from a peridotite breccia, evidently produced by exceedingly violent volcanic explosions which shattered the peridotite and mixed with it a large proportion of fragments derived from the adjacent country [native] rocks.”  Generally, experts think ground water aided the explosion, but some minimize that effect and emphasize the role of carbon dioxide originating in the mantle (e.g., Lionel Wilson and James W. Head, “Diatremes  and Kimberlites 2: An Integrated Model of the Ascent and Eruption of Kimberlitic Magmas and the Production of Crater, Diatreme, and Hypabyssal Facies,” 8th International Kimberlite Conference Long Abstract, January 2006, online at: http://www.dtm.ciw.edu/ftp/shirey/gabi/GabiTrash/ABSTRACTS/FLA_0244.PDF ).

    Crookes, himself, considered water, or steam, a vital part of the eruptive force, for “steam in the presence of molten or even red-hot iron liberates large volumes of hydrogen gas, together with less quantities of hydrocarbons of all kinds.  Erosion commenced by steam would be continued by the other gases.  It would be easy for pipes, large as any found in South Africa, to be scored out in this manner” (ibid.).

    Compare the above with the conclusion in John Sinkankas’ popular guidebook, Gemstones of North America, Vol. 3 of Gemstones of the World Series (Princeton:  D. Van Nostrand Co., 1959), 30:  “Because of the cylindrical shape of diamond-bearing bodies in South Africa–the so-called ‘pipes’–it was thought that kimberlite was volcanic in origin.  This theory is now discredited since little evidence has been advanced to bear out its claims.  It is now believed that kimberlite, like other peridotites, is a deep-seated rock allied to granite and not a product of eruption.”

[3] For the most accurate surface map, see Miser and Purdue, USGS Bulletin 808, Plate 10.  The map and Miser’s analysis also appeared in several earlier editions of the Bulletin authored by Miser (geology) and Clarence S. Ross (petrography).  Their initial publication:  “Diamond-Bearing Peridotite in Pike County, Arkansas,” Economic Geology, 17 (December 1922), 662-674.  Miser’s first, brief report, “New Areas of Diamond-Bearing Peridotite in Arkansas” (1914), 534-546, preceded the surveying and mapping of the big formation near Murfreesboro, which was completed in 1916.

    Miser referred to the formation as the “Prairie Creek Area of Peridotite” because it lay immediately east of Prairie Creek, near the junction of the creek and the Little Missouri River.  As Miser and others noted, a peridotite dike cut across Prairie Creek.

    Miser classified the heart of today’s recreational diamond hunting area as “volcanic breccia,” a distinctive mixed material which usually turns dull green when weathered.  The hunting area, comprising most of the current plowed field, also extends over into the wide drainage ditch to the west because diamonds wash there as the volcanic breccia erodes (the broad drain is depicted as a series of contour lines jutting northward toward East Hill, in the middle of the USGS map).  The two other categories of material, concentrated on the west half of the formation, were classified as “intrusive peridotite” (basically hard, dark gray rock) and “tuff and fine-grained breccia” (hard, light blue).

    Into the mid 1970s, studies generally accepted the USGS’s three categories, but refined the analysis somewhat.  Richard D. Lewis, for instance, separated the diamond-bearing breccia into three varieties and suggested they represented different phases of volcanic activity (Lewis, “Mineralogy, Petrology and Geophysical Aspects of Prairie Creek Kimberlite Near Murfreesboro, Arkansas” [Purdue University, 1977], 21, 33ff.).  Lewis referred to the dark intrusive as “micaceous peridotite” and the light-blue as simply a sedimentary tuff.  Differing with the USGS and others who concluded that the volcanic breccia was deposited after the intrusive peridotite, he found the opposite (22; “Summary and Conclusions,” 108-111).  Lewis’s bibliography of previous studies is comprehensive (112-117).

    In the late 1970s geologists began redefining the pipe as “lamproite” rather than kimberlite.  See infra, “Promotional Maneuvering and a Final Test.”

[4] Reports on the formation appeared in 1842 (W. Byrd Powell, who merely linked it to other igneous deposits in the area), 1846 (C. U. Shepard, who offered a vague description of the rock), and 1860 (D. D. Owen, who added “porphyritic greenstone” to the analysis).  J. Francis Williams, “The Igneous Rocks of Arkansas,” Annual Report of the Geological Survey of Arkansas, 2 (Little Rock: Woodruff Printing Co., 1891), 377-378, acknowledged the three men’s contribution.

[5] Before 1906, peridotite and related serpentine formations were studied from the Great Lakes region down into North Carolina.  Peridotite was still used predominantly in the early 1900s, but kimberlite steadily entered the vocabulary.  John F. Fuller, “Diamond Mine in Pike County, Arkansas,” Engineering and Mining Journal, 87, No. 3 (January 16, 1909), 153, for instance, referred to Henry Carvill Lewis’s previous description:  “‘. . . a porphyritic volcanic peridotite of basaltic structure,’ which is named kimberlite.  This peridotite, or kimberlite, is more popularly known in South Africa as ‘blue ground.’”  Miser and Ross, “Diamond-Bearing Peridotite,” 672, referred to both types of peridotite found in South African mines:  “a porphyritic basaltic kimberlite rich in olivine and poor in mica” and “a porphyritic mica peridotite of lamprophyric habit” (occurring in dikes and sills as well as some pipes).

    For concise comment on the varieties of blue ground, see Fuller, ibid.  The description is largely repeated in Fuller, Report to L. F. Loree, President, Delaware & Hudson Railroad Co., New York, June 25, 1908, in “Reports and Information Gathered by John T. Fuller From 1908 to 1931, Diamond Fields in Pike County, Arkansas,” 12 (see Bibliographic Note, infra).

[6] John C. Branner and Richard N. Brackett, “The Peridotite of Pike County, Arkansas,” American Journal of Science, Series 3, 38, (1889), 50-59.  J. Francis Williams, “The Igneous Rocks of Arkansas,” 377-391, virtually reprints Branner and Brackett.

   John Casper Branner became State Geologist in 1887 and served five years.  An eminent professional with extensive experience in Brazil, he was elected president of the American Geological Society in 1904.  He died March 1, 1922 in California.  For an excellent sketch:  The National Cyclopedia of American Biography, 24 (James T. White & Co., 1935), 278-279.  Ibid., XIII (1906), 599-600, recognized his distinguished career up to that point. 

[7] John C. Branner, “Correspondence and Discussion:  Some Facts and Corrections Regarding the Diamond Region of Arkansas,” Engineering and Mining Journal (EMJ ), 87 (February 13, 1909), 371-372.  Branner noted EMJ coverage of the Arkansas frenzy and also listed a related Arkansas Gazette article.

    George F. Kunz, generally recognized as the nation’s foremost geologist-mineralogist, confirmed Branner’s explanation:  Kunz, “Diamond Mine in Pike County, Arkansas,” EMJ, 87 (May 8, 1909), 963.

[8] Branner described the frenzy in Kentucky, where he served as a consultant before going to Arkansas (ibid., 371).  Commenting on diamond exploration in Kentucky after the rush, Kunz said that after years of effort, “much money has been spent without finding a single diamond” (ibid.).  Austin Q. Millar, a central actor in the Pike County fields, was still testing for diamonds in Kentucky when he left for Arkansas in 1908 (infra, “The Millars and the Kimberlite Company”).

    The diamond swindles and “false alarms” of the ‘70s and ‘80s have gotten much attention in the literature.  The biggest “salting” swindle occurred in the West, when prominent California businessmen and others lost a huge sum of money.

[9] An article, “Genuine Diamonds–Southwest Arkansas Gives a Diamond to the World,” Arkansas Gazette, April 7, 1876, p. 1, told of two men who visited the Gazette and displayed a one-carat cut diamond set in a ring.  The enthusiastic writer concluded:  “The diamond is not the property of Mr. Williamson [the bearer], but is owned by a friend, at whose request the place where found is kept a profound secret.  It is a ‘big bonanza,’ and we trust that Arkansas will show up many more of the precious stones.”