Cleavage is the splitting of a gemstone along the direction of its crystal faces where molecules have weak bonding. This can occur only in crystalline minerals when a precise blow is given in a particular direction, even if the habit does not show the crystal faces. The result of cleavage is a more or less flat plane with often a silky luster.
There are several directions of cleavage.
- Prismatic cleavage
- Basal cleavage
- Pinacoidal cleavage
- Octahedral cleavage
- Rhombohedral cleavage
The quality of cleavage is expressed with a few simple phrases.
Parting is the splitting of a gemstone along twinning planes.
One of the most profound examples of this is mica which concists of many very thin layers that can be easily parted with the fingernail. It is this mineral that is traditionally used for quarter wave plates.
Fracture is the random breakage of a gemstone when behing hit with a sharp or hard object. The most common type of fracture is termed "conchoidal" (shell-like).
The types of fracture are: