Cleavage planes

From Conservapedia

Cleavage planes are common in crystalline minerals. They tend to follow weaker bonded lines through the crystal matrix, and lend a characteristic appearance to many minerals when they are fractured naturally or by a mineralogist.

The art of diamond cutting exploits the many possible cleavage planes in the diamond crystal in order to create a beautifully refractive gem from the raw material.

Graphite, interestingly another pure carbon crystal, is so weakly linked between its layers that it is used as a lubricant, since the layers slide readily over one another.