Uranium. Source: Wikipedia

Uranium (chemical element U, better known as U3O8) in its pure form is a silvery white metal of very high density -- even denser, than lead. Uranium can take many chemical forms, but in nature it is generally found as an oxide (in combination with oxygen). Triuranium octoxide (U3O8) is the most stable form of uranium oxide and is the form most commonly found in nature.