What is Cleavage in Stone?
Cleavage is a straight crack which has the tendency to break crystal or gemstone at certain points or planes. It is must for gem cutters and gem buyers to know about cleavage in stones. Cleavage usually occurs in gems and minerals where there is weaker atomic bonding the material.[caption id="attachment_2435" align="aligncenter" width="260"] Cleavage in Flourite[/caption]
Cleavage is a straight crack in gemstones and minerals.
Facts about Cleavage in Gems and Minerals
- It occurs in a crystalline material.
- It can be both internal and external
- The direction of splitting is dependent upon the internal crystal structure. In some points, atoms are closed together and in strong bonding. In some areas, atoms are in weak zones.
- Cleavage cannot occur in amorphous material as glass. In amorphous material, there are no structural planes which can define splitting to be taken place in. Glass can only be broken through the fracture.
- A single cleavage cannot occur example is jade.
- In diamonds, all the bonds are equal in straight but they are relatively weaker zones parallel to a certain area.
Cleavage can be further defined in terms of quality and directions of gemstones and minerals.
Perfect: broad flat, shiny
Good: less flat surface
Fair to Poor: Seldom visible in gem quality stones
Cleavage also helps in gem identification. Cleave may reflect or block the light due to internal reflection and one may see a crack or mirror-like flat inclusion. Cleavage is not extended throughout the crystal called incipient cleavage, It may show pearly luster e.g topaz.
Advantage / Disadvantage of Cleavage in Mineral Gemstones
A gemologist can identify cleavage in the rough and even cut. It helps in mineral gemstones identification.
- Hardness direction can be found in diamond due to cleavage
- facts cannot be pointed towards cleavage
- It is necessary to cut facets at a slight angle to the cleavage plane.
- In some cases, incipient cleavage maks the stone weaker
- It can destroy stone during thermal shock or blow (pressure) heat, sudden stress while cutting or polishing or when repairing.
- Cleavage can affect the durability of stones.
List of Gemstones Usually having Cleavage
Not all gems have cleavage but many have. Some of the gems names are hereunder:-