The rough surface of the tile grinds the edge of the mineral to a powder. The color of the powder may be different from that of the solid piece. Luster is another characteristic to be observed. The nonmetallic minerals may appear glassy, greasy, silky, or pearly. Cleavage is a test to determine the way a mineral splits in planes.
Cleavage is related to the mineral's crystal structure.
Mica, for example, splits in thin sheets. Some minerals break into sharp, splintery pieces called fractures. Every mineral has a particular specific gravity, or density. It may be determined by weighing the mineral in the air and then suspended in water. The weight in the air, divided by the difference in weight while in water, is the specific gravity.