English: Kimberlites and lamproites have significant economic importance because they are host rocks for gem-grade and industrial-grade diamonds. Kimberlites and lamproites are unusual igneous bodies having overall pipe-shaped geometries. Their mode of formation is only moderately understood because they have not been observed forming. These rocks are known from scattered localities throughout the world - only some are significantly diamondiferous. The earliest diamond localities were India and Brazil. Africa was later discovered to have many kimberlites and is world-famous for producing large numbers of diamonds. Other notable diamondiferous kimberlite-lamproite occurrences include Russia, China, northwestern Australia, and northwestern Canada.

Kimberlites are named for the town of Kimberley, South Africa. Several kimberlite pipes occur in the Kimberley area. Kimberlites have a gently tapering-downward, pipe-shaped cross-section. Lamproites have a cross-section more closely resembling that of a martini glass.

The locality seen here is the only publicly-accessible diamond collecting site in America - Crater of Diamonds State Park, in Arkansas. At the site, diamondiferous lamproitic rocks of the Prairie Creek Lamproite are exposed (actually, it’s just a field of rocky dirt). Diamonds are found regularly by tourists. The Prairie Creek Lamproite is an ultramafic pipe that intruded Proterozoic-aged (Grenvillian) basement rocks. Prairie Creek rocks are Cretaceous in age and date to about 106 million years old.

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From park signage:

THE “CRATER” PIPE The “Crater” diamond deposits are the result of a series of volcanic eruptions that carried a diamond-bearing magma called peridotite to the surface. This molten rock slowly cooled, forming a solid, vertical shaft or pipe.

Later, secondary eruptions occurred that shattered much of the peridotite. The fragmented mixture left by the eruptions formed a volcanic breccia that comprises the majority of the searching area. The intrusive peridotite, and a tuff and fine-grained breccia, also formed by the eruptions, cover the remainder of the mine surface.

STRATIGRAPHIC PROFILE The volcanic eruptions forming the “Crater’s” pipe had to force their way upward through the layers of rock and sediment formed during Paleozoic and Mesozoic times.

Diamonds are located throughout the pipe - from the surface to an undetermined depth (possibly several thousand meters). The mine is simply the exposed surface area of the pipe.

TOPOGRAPHY Surface features at the “Crater” today reflect the depositional, erosional, and weathering processes which have occurred since the pipe’s formation. Erosional forces, such as wind and water, have erased the original surface features of the pipe and surrounding rock formations.

Most of the exposed volcanic rocks tend to weather rapidly forming soils of various colors. It is in these soils that we find diamonds today.

In 1948, Glen L. Martin leased property to build a diamond processing plant. Martin’s plant operation lasted only eight months after recovering less than 250 carats of diamonds. The original location of the Martin Plant has been preserved. Its location lies just south of the Visitor Center.

DAZZLING DIAMONDS FROM THE CRATER Over 85,000 Crater diamonds have been found. Each year an average of 600 are found. 1924 - The Uncle Sam, 40.23 carats 1956 - The Star of Arkansas, 15.33 carats 1956 - The Eisenhower, 6.11 carats 1960 - The Gary Moore, 6.43 carats

Over 24,000 diamonds have been found since Crater of Diamonds State Park was established in 1972. 1975 - The Amarillo Starlight, 163.7 carats 1978 - The Lamle Diamond, 8.61 carats 1981 - The Star of Shreveport, 8.82 carats 1986 - The Conell Diamond, 7.95 carats 1990 - The Strawn-Wagner, 3.03 carats 1997 - The Cooper Diamonds, 6.72 & 6.0 carats 1998 - The Dickinson/Stevens, 7.28 carats

An end to a mystery Since 1906, when John Huddleston discovered the first diamonds in this field, people have wondered what was beneath this green soil surface. Just how deep and how large is this diamond deposit? From 1990 to 1997, Arkansas State Parks and several international commercial mining companies conducted tests to determine the underground size and shape of the diamond deposit, and to evaluate the grade of diamonds from the mine. Phase I testing determined that this is the world’s 8th largest diamond deposit, in size and shape. It extends over 660 feet into the earth. In phase II, 210 diamonds totaling 45.7 carats were recovered. This verified that a significant number of diamonds occur within this large deposit. However, the grade was less than commercially profitable at the time and interest in further testing diminished.

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Geologic unit: Prairie Creek Lamproite (Prairie Creek Pipe), mid-Albian Stage, late Early Cretaceous, ~106 Ma

Locality: Crater of Diamonds State Park, just south of the town of Murfreesboro, Pike County, southwestern Arkansas, USA (34° 01’ 55” to 34° 02’ 05” North latitude, 93° 40’ 18” to 93° 40’ 25” West longitude)