Gilsonite has the appearance of obsidian or anthracite coal. Gilsonite (a.k.a. uintaite) is solidified oil. ItŐs an asphaltite - a naturally solid hydrocarbon - that is jet black in color, very lightweight, brittle, and has a conchoidal fracture. The largest ŇdepositsÓ of gilsonite in the world are in the Uinta Basin of northeastern Utah, USA. Gilsonite occurs there as vertical NW-SE trending veins intruded within the Uinta Formation (Eocene). The source of the hydrocarbons is organic-rich beds in the underlying Green River Formation (Eocene) and Wasatch Formation (Paleocene).
Gilsonite (uintaite) (4.5 cm across) from northeastern Utah.
Not all gilsonite material from Utah has an obsidian-like appearance. The sample shown below is a variety called pencillated gilsonite, characterized by having abundant parallel fractures. The fractures give the material a splintery appearance (from certain angles, the stuff looks like hornblende schist). The only pencillated gilsonite material I've seen also has abundant fracture circlets (conchoidal fracture) - see below.
Pencillated gilsonite (uintaite) (~4.3 cm across) from northeastern Utah. This is a commercial sample mined from the Cottonwood Dike (Cottonwood Vein), a N60-64W striking, 13 to 16 km long, 0.6 to 0.9 meter wide, vertical to subvertical gilsonite dike in the Willow Creek System. The dike was probably emplaced ~10 to 30 million years ago. The Cottonwood is mined south-southeast of Ouray (section 35, T10S, R21E, Big Pack Mountain NE 7.5' USGS topographic quadrangle) in south-central Uintah County, Utah.
Pencillated gilsonite sample & locality info. generously provided by Jim Lekas of the Lexco Gilsonite Company.