Book 27, Question 8
Why do some crystals split along flat lines?
Inside a crystal, atoms are held together more weakly in some directions than in others.
When the crystal is tapped or hit, it can split along those weak flat planes. This is called cleavage. Mica splits into thin, bendy sheets, and calcite breaks into slanted blocks. Gem cutters study cleavage so they can split big stones the safe, smart way.