Grinds materials not readily reduced in ceramic or metal mortars

Made of a synthetic sapphire substance composed of fine-grained crystals of high-purity aluminum oxide held together by a high strength, non-metallic, vitreous bond. Smooth, but not glossy, hard, pitless grinding surface. Inert to most corrosive chemicals and impervious to liquids under vacuum. Specific gravity, 3.6. Hardness, approximately 9 on Mohs scale.