Uncut rough diamonds look like water-worn quartz pebbles, but they can be differentiated by properties such as crystal structure, specific gravity, hardness, and other distinguishing qualities. Diamonds have an isometric crystal structure, a specific gravity of 3.1–3.5, a Mohs hardness of 10, cling to a grease table, and glow under shortwave ultraviolet light in some situations. A combination of these traits is used to correctly identify an uncut rough diamond.