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Diamonds, Kimberlites, and Lamprophyres

Two types of rocks in the world are known to contain diamonds — kimberlites and lamprophyres — both of which are dark, ultramafic igneous rocks. South Africa, Canada, India, China, and Russia contain some of the most famous diamond-bearing kimberlite localities, and Australia contains rare diamond-bearing lamprophyres. In the United States, Arkansas, Colorado, and Wyoming contain kimberlites that have produced diamonds.

Kentucky contains both of these type of rocks — kimberlites in Elliott County and lamprophyre dikes in western Kentucky — although no naturally occurring diamonds have ever been found in either of these rocks. Diamonds derived from glacial outwash or drift are possible in parts of northern Kentucky, since the glacial outwash sediments are derived from igneous rocks, possibly including kimberlites and lamprophyres, which would have originated in the northeastern United States and Canada, but such occurrences would be very rare and difficult to find.

A diamond was found in Adair County, but its source is unknown; thre are no kimberlites or lamprophyres in the area and glaciers did not extend that far into Kentucky. Another diamond was found in West Virginia close to the Kentucky and Virginia borders, but it was determined to have originated in Virginia.

The Elliott County kimberlites have been explored extensively for diamonds. Numerous small operations have examined the area since the 1940's, and core samples were obtained and a pilot mining operation was conducted in the 1950's and 60's. More recent bulk sampling and analysis was conducted in the 1990's. Geologic reports on the area confirm that the dikes are kimberlite and some geophysical work was conducted (Boliver, 1982). No authenticated diamonds have been found at this location, despite the extensive work in the area. Some evidence of mining activity and an old log washer are still visible on the property. This property is on private land, so landowner permission should be obtained prior to inspecting the property.

Log washer on kimberlite dike, Elliott County, Ky.

Diamonds have been found in lamprophyre dikes in Australia, but they are very rare and diamonds are not expected to occur in the Kentucky lamprophyre rocks. Some exploration activity has occurred on lamprophyre dikes in western Kentucky in Livingston and Crittenden Counties. Bulk sampling and caustic dissolution was conducted on one dike, but no diamonds were recovered. Researchers at the Kentucky Geological Survey and University of Kentucky Department of Earth and Environmental Sciences are currently investigating these dikes to determine their petrology and mineral chemistry.