a hard, very heavy, dark-colored, radioactive mineral, UO, the chief ore of uranium; uranium oxide
See uraninite in American Heritage Dictionary 4
(yo͝o-rāˈnə-nītˌ)
noun
A complex brownish-black mineral, UO2, forming the chief ore of uranium and containing variable amounts of radium, lead, thorium, and other elements. It is isomorphous with thorianite.