Cleavage is the splitting of a gemstone along the direction of its crystal faces where atoms have weaker bonding. This can occur only in crystalline minerals when a precise blow is given in a particular direction. The result of cleavage is a more or less flat plane with often a silky luster.
Cleavage is a reproducable property of a gemstone and can be done at any point of the cleavage direction.
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 or pressure planes.
Although this is similar to cleavage, parting can only be done along certain, well defined, planes of weakness and it is not reproducable like cleavage. Usually these parting planes are created during growth of a crystal when pressure was applied to the crystal causing it to (partially) break or fracture. Later overgrowth then heals the fracture, leaving a weak area inside.
One of the most profound examples of parting 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 along a direction other than then the direction of cleavage or parting. The most common type of fracture is termed "conchoidal" (shell-like).
The types of fracture are:
- Gemmology (2005) - Peter Read