Many minerals have a tendency to tarnish, so a freshly broken surface may have to be exposed to reveal the true colour. The rough surface turns a part of the mineral into very fine powder, whose colour may be different from that of the mineral itself and much more characteristic. Some hard minerals must be pounded with a hammer to produce a powder. All crystalline minerals cleave or have cleavage when they break along smooth surfaces in definite directions. The specific pattern of atoms in layers, whose cohesion differs from one to another, produces this feature.
Cleavage is described according to the crystal direction and how easily it is obtained.
The manner in which minerals break in irregular directions is described as a fracture. The type of fracture depends on the appearance of the new surface. For example, a series of arcs, like a shell, is called a conchoidal fracture. Other samples of fracture include hackly, even, uneven and earthy. The weight of a substance in proportion to the weight of its own volume of water is its specific gravity.