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.