So the color of a powdered mineral is considered much more diagnostic. An easy way to do this test is to examine the color of the mineral's streak when scratched on a porcelain plate. The external form or shape of crystals may be the result of fracture or of growth . If a crystal has a plane of weak chemical bonds it can break along perfect planes. This is called cleavage .
Mica has a single perfect cleavage whereas calcite has three.
A Mineral Structures and Properties Data Base Descriptions and List of Mineral Specimens Rocks A rock is an aggregate of minerals. Rocks can be formed by many different processes. Rocks that are formed by crystallization of a melt are igneous. These are generally classified by the size of the particles, although the compositions change systematically with particle size. Rocks that form by recrystallization in the solid state are metamorphic.