Uraninite is uranium oxide and an important ore of uranium. It is a black, brittle material, seldom occurring in crystals. Its usual occurrence is as masses or disseminated in rocks. Weathering creates alteration products that are generally yellowish or reddish in color. Radium was first discovered in uraninite, and the mineral's natural radioactivity will expose photographic film much as light does. A slice of uraninite placed on a sheet of film will produce a 'self-photograph', called an 'autoradiograph'. Grafton, New Hampshire is a well-known locality for this mineral.