Wikimedia
Wikipedia: Oil Shale
When converted to petroleum, oil shale is a valuable resource produced by many countries around the world, including the U.S. Wikipedia discusses the geology, history, economics, and environmental considerations of oil shale.
Wikimedia
Wikipedia: Shale
Originating as clay or mud, shale is a sedimentary rock formed by deposits of small particles by slow moving water. Wikipedia includes color images and discusses composition and deposition.