UO₂ A black, brownish-black, or dark-brown radioactive mineral that is isometric in crystallization; often contains impurities such as thorium, radium, cerium, and yttrium metals, and lead; the chief ore of uranium; hardness is 5.5-6 on Mohs scale, and specific gravity of pure UO₂ is 10.9, but that of most natural material is 9.7-7.5. Also known as coracite; ulrichite.