Difference between revisions of "Cleavage"

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==Cleavage==
 
==Cleavage==
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.<br />
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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.<br />
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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.
 
There are several directions of cleavage.
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Parting is the splitting of a gemstone along twinning or pressure planes.<br />
 
Parting is the splitting of a gemstone along twinning or pressure planes.<br />
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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.
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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 [[Polariscope#Quarter_wave_plates|quarter wave plates]].
 
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 [[Polariscope#Quarter_wave_plates|quarter wave plates]].
 
==Fracture==
 
==Fracture==
  
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).<br />
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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).<br />
  
 
The types of fracture are:
 
The types of fracture are:
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* uneven
 
* uneven
 
* even
 
* even
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==Sources==
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* ''Gemmology'' (2005) - Peter Read
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==External links==
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* [http://www.galleries.com/minerals/property/parting.htm Parting]
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* [http://www.galleries.com/minerals/property/cleavage.htm Cleavage]

Revision as of 07:37, 17 December 2006


Cleavage

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.

  • Perfect
  • Good
  • Fair
  • Poor
  • None
Prismatic cleavage
Basal cleavage


Parting

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

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:

  • conchoidal
  • uneven
  • even

Sources

  • Gemmology (2005) - Peter Read

External links