What is cleavage of a Crystal?: Key to Gemstone Identification

What is cleavage of a Crystal?: Key to Gemstone Identification

Today, our focus will be on " What is cleavage in a gemstone? This fruitful insight will help the gem cutters and jewelers in gemstone identification and the assessment of gemstone quality.

What is cleavage of a crystal and Gemstone?

Cleavage is actually a linear fracture that cause a crystal or gemstone to break alongwith the specific points or angles. It is essential for both gem cutters and buyers to understand about the cleavage. They often arise in gems and minerals with weaker atomic bonds within the material. Cleavage is a straight crack in any gems or crystal.

Facts about Cleavage in Gems and Minerals

Some facts about cleavage of gems and crystals. Have a look at them.

1- Crystalline Nature:

  • Cleavage occurs in a crystalline materials.
  • it can be found inside Internal and External both side of the crystals
  • Cleavage can manifest both internally and externally on a crystal.

2- Dependence on Crystal Structure:

  • The internal crystal structure determines the direction of the cleavage inside the crystal.

3- Variation in Atomic Bond Strength:

  • Cleavage also comes due to weak or strong bonding of atoms within the crystal structure. In some areas, atoms are close enough to make strong bond. Weak bonding let the cleavage appear inside the stone.
  • Weak zones of atomic bonding let the cleavage apears and leting the stone splitting into two pieces. 

4- Inability in Amorphous Materials:

  • Amorphous materials such as glasses lack well defined structural planes for cleavage. They exhibit fractures in such materials. 

5- More than one cleavge

  • Some stones can have more than one cleavage. Jade is an example. 

6-Cleavage in Diamond Stone

  • While diamonds possess strong bonds in all directions, there are relatively weaker zones parallel to specific areas, enabling cleavage.

Cleavage can be further defined in terms of quality and directions of gemstones and minerals such as below:-

Grading of a Stone keeping in Mind cleavage

  1. None: no Cleavage
  2. Poor: lot of fractures
  3. Good: less flat surface
  4. Perfect: broad flat, shiny
  5. Fair: Seldom visible in gem quality stones

The presence of cleavage helps in playing significant role while identifying a gemstone or crystal. It tells us that the light interacts within the gemstone. When a gem has a cleavage, it can either reflect or block light due to internal reflection. These reflections further reveal other multiple characteristics such as cracks or mirror-like flat inclusions. Sometimes, cleavage isn't uniform throughout the crystal. This type is called incipient cleavage which may cause a pearly luster, as usually seen in topaz.

What are Advantage / Disadvantage of Cleavage in Mineral Gemstones

A gemologist can identify the cleavage in both rough gemstone and cut gemstones. Here are some advantages and disadvantages of cleavage in a crystal or a loose gemstone. 

  • Cleavage shows the direction of hardness in the diamonds
  • Facets cannot be pointed toward cleavage.
  • It is necessary to cut the stones and make facets at a slight angle to the cleavage plane.
  • Sometimes, an Incipient cleavage can weaken the stones.
  • Cleavage can split the stones during exposed to different environments such as thermal shock or sudden stress. These shocks can occur during cutting, polishing, or repairing. So it is must for gemstone cutters to know about weaker zone of the stone in order to avoid splitting the stone. 
  • A cleavage can affect the durability of the stone. 

Tips for Gemstones Cutters about Cleavage of a Stone

  1.  It is always beneficial to knowabout gemstones names which have mostly cleavage or fractures. It will help them to cut on the points with strong zone inside the stone.
  2. Point out by marker where external cleavage is visible
  3. Use loupe to see inside the points where cleavage can be possible.
  4. Use maximum precautions to cut the stone which are brittle by nature such as emerald, aquamarine, morganite 
  5. Try to avoiding exposure of the stone towards shocks such as instant heating, pressure or chemicals

List of Gemstones Usually having Cleavage

Not all gems have cleavage but many have. Some of the gems names are hereunder:-

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