Lesson 2: The Geosphere - We learned the physical properties used to identify a mineral. The way the surface of a mineral reflects light is called luster. If you scratch a mineral across a tile, it will leave powder, the color of which is called streak. Minerals are ranked by their relative hardness on Mohs scale. The tendency of minerals to break along smooth faces is called cleavage. The tendency of minerals to break along irregular surfaces is called fracture.