Cleavage

What is cleavage?
Cleavage is the ability of a mineral to break along preferred planes.
Some minerals show different colors along different crystallographic axes. This is known as pleochroism.

How is cleavage described?
Minerals that have "perfect" cleavage almost always break in a preferred direction. Minerals that have "good" cleavage will sometimes break in a particular direction, and other times, they may not.