(~7.4 centimeters across at its widest)

Kimberlites and lamproites have tremendous economic importance because they are host rocks for gem-grade and industrial-grade diamonds. Kimberlites & 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. Kimberlites & lamproites are known from scattered localities throughout the world - only some are significantly diamondiferous. Classic localities for diamonds are India and Brazil. Africa was also 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.

Seen here is Ohio's only known kimberlite sample. It's not from Ohio's bedrock, however - it's a modern fluvial clast, derived from a Pleistocene glacial till. The original kimberlite body was somewhere in Canada. Pleistocene glaciers eroded much of Canada's bedrock and delivered the debris to the northern parts of America, including about two-thirds of Ohio. Because this sample was not collected in-situ, additional specimens are unlikely to be found at the same site. Kimberlite is a rare rock in general.

Mantle xenoliths composed of peridotite (= an olivine-rich, ultramafic, intrusive igneous rock) are scattered throughout this specimen (click on the photo once or twice to zoom in; a small peridotite xenolith is below & to the right of the picture's center). No diamonds are visible at the surface.

If anyone is preparing to counter my above statement that this is Ohio's only known kimberlite with a citation to Baranoski et al. (2007), there's a tale there. Long story short - it was a practical joke gone horribly wrong.

Long story long - The Murray # 2-1875 was a petroleum well drilled in 1994 into the Dillon Falls Astrobleme northwest of Zanesville, Ohio. There were two stages of drilling: 1) into the structure, which dates to the Early or Middle Ordovician; and 2) almost to basement rocks. Several wells have been drilled into the sandstones that fill the inferred impact crater - some have produced natural gas. During the second stage of drilling, while penetrating Cambrian dolostones (= Copper Ridge Dolomite to Rome Dolomite succession), a geologist, as a joke, salted a well cuttings sample bag with dark-colored, crystalline-textured igneous rock chips. The intent was to mess with a couple of the other geologists at the well site. The mudlogger washed the sample and presented it to the geologists, who were shocked. Different interpretations were quickly formulated - a sill? a kimberlite? This resulted in much commotion and laughter. The sample bag was tossed into a pile of other bags. All the sample bags were eventually sent to the Ohio Geological Survey. The geologists forgot all about it - just a small practical joke. Until 2007 - it was published! Baranoski et al. (2007) described the occurrence of an odd igneous body in this well - possibly a kimberlite. They did note the complete lack of evidence for the inferred igneous body on the wire-line log. Oops. One of the original geologists at the well site contacted the Survey upon learning of the publication. They were informed that it was a practical joke - all in fun. It was never intended to mislead anyone, let along the Ohio Geological Survey. Despite being told about the situation, the publication has not been retracted or corrected (as far as I know). Everyone gets fooled sometimes. I have. But boo-boos need to be corrected. This is a major boo-boo.

Locality: loose clast from a modern gravel bar (derived from Upper Pleistocene glacial till) along a south-flowing tributary of Clear Fork, just north of Chatham Road bridge, western side of the town of Chatham, northwestern Newton Township, northern Licking County, central Ohio, USA

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