Oil shale is a source rock for petroleum that has not had the chance to "ripen" into crude oil. Thus it is distinctly different from oil sand, in which petroleum has migrated into a porous rock bed. Oil shale may be burned directly or processed with heat, either underground or in a processing plant, to drive off synthetic oil and gas. Oil shale was widely exploited in the early 20th century until petroleum undercut its price. The United States has the world's largest reserves; learn more from the U.S. Department of Energy.
This hand specimen of oil shale is from the Grube Messel Ölschiefer (oil shale) of Germany, famous as a lagerstätte fossil site.