Cleavage is one characteristic of minerals that refers to the tendency to split along planes of weakness.
The space between a woman's breasts is her cleavage. When women wears low-cut...
Biotite (black mica) and muscovite (white mica) are both minerals that have perfect basal cleavage--one direction. It cleaves into thin sheets. Feldspars (albite, oligoclase, andesine, labradorite,...