English: Gilsonite pahoehoe paralava from the uppermost Holocene of Utah, USA. (public display, Utah Field House of Natural History, Vernal, Utah, USA)

This is one of the most remarkable rare rocks I've ever encountered. This is gilsonite (a.k.a. uintaite), which is a variety of asphaltite - it's a solid hydrocarbon - basically solidified oil. Gilsonite usually has the appearance of obsidian or anthracite coal. It is jet black in color, very lightweight, brittle, and has a conchoidal fracture. The largest “deposits” of gilsonite in the world are in the Uinta Basin of northeastern Utah, USA. Gilsonite occurs there as vertical NW-SE trending dikes (veins) intruded within the Eocene-aged Uinta Formation. The source of the hydrocarbons is organic-rich beds in the underlying Green River Formation (Eocene) and Wasatch Formation (Paleocene).

Gilsonite is not readily combustible and is not a fuel, but it does have economic value. Gilsonite is mined in northeastern Utah and processed and is used in numerous products and applications (e.g., see: www.geospectra.net/kite/gilsonite/gilson.htm).

The specimen shown above formed when a lightning-induced wildfire in summer 2012 heated and melted gilsonite in old mine piles. The molten gilsonite (a type of paralava) flowed, cooled, and solidified back into gilsonite. While molten, it developed a smooth to ropey top surface, which is essentially identical to the basalt pahoehoe lava flows of Kilauea Volcano at the Hawaii Hotspot. These gilsonite pahoehoe paralavas reportedly weathered relatively quickly, but this specimen was collected fresh.

Locality: gilsonite paralava flow in the Wolf Den Fire area, southwest of the historic town of Dragon, southeastern Uintah County, northeastern Utah, USA

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Some info. provided by Roland Heath.