A scale of relative hardness, Moh's hardness scale, lists the hardness of 10 minerals and some everyday objects. The hardness of an unknown mineral is discovered by determining whether or not it will scratch the minerals and objects on the list. The tendency for a mineral to break along predefined planes of weakness, usually forming smooth surfaces. The weak planes are controlled by the internal structure of the crystal. Not all minerals possess cleavage, only those which break in specific directions on relatively smooth planes.
Some minerals have more than one direction of cleavage.
This describes the external shape of a crystal. The color of powdered particles of a mineral. The powder is produced by rubbing the mineral on a white, porcelain plate. This only works with minerals of hardness Specific Gravity Related to the density, or weight per unit volume of a mineral. Some minerals feel heavier than expected for their size.