Many kimberlite pipes also produce rich alluvial or eluvial diamond placer deposits. These diamonds probably formed in kimberlite magma and sampled the volatiles. For instance, it occurs in uncommon mantle xenoliths, carried up by kimberlite. Two Jurassic kimberlite dikes exist in Pennsylvania. They found traces of the volcanic rock kimberlite, which is usually associated with diamonds. The Ekati and Diavik mines are actively mining kimberlite diamonds. Natural diamonds occur in the rock kimberlite, found in ancient volcanic "necks", or "pipes". Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes. Diamond prospectors continue to search the globe for diamond-bearing kimberlite and lamproite pipes. The kimberlite eruptions then bring the diamonds from over depth to the surface. No diamonds have been found in the Kansas kimberlite occurrences. In reconnaissance for diamond mining, blue ground refers to a layer of non-oxidized kimberlite. Uplifting events caused the kimberlite to be exposed. No kimberlite has erupted in human history. De Beers started looking for Kimberlite Pipes within Canada in the 1960s.