What is Mineral Cleavage?

Under ideal circumstances, a cleavage plane might be virtually smooth and flat on an atomic scale. A break in a mineral that doesn't occur along cleavage planes is either a fracture or a parting.

Mineral Cleavage

Since the atomic arrangement within a crystal is symmetrical, the planes of specific bonds are symmetrically disposed within the crystal. Internal cleavage planes are, therefore, as symmetrical as external crystal faces.

The term cleavage in mineralogy only applies to crystalline materials because it is related to the orderly arrangement of atoms in the crystal lattice. Non-crystalline materials, such as glasses or amorphous substances, do not have cleavage because they lack a regular atomic arrangement and, instead, break in irregular ways (called fracture). Stated differently, since glass is a super-cooled liquid in which the atoms are not arranged in a long-range periodic array, it can have no cleavage because there are no uniform bond layers.

Cleavage Grades

Mineral cleavage is usually described with reference to crystallographic axes and directions and is also graded according to its "perfection," or ease with which the gem can be spilt along the cleavage.

The five grades or degrees are:

• None
• Poor (or weak)
• Fair (or moderate)
• Good (or imperfect)
• Perfect

Minerals with perfect cleavage are easiest to split, while those with no cleavage are hardest to split.

Variable Cleavage Within a Single Crystal

Sometimes, there are different degrees of cleavage perfection in different directions within the same crystal.

Can Cleavage Affect Faceting and Jewelry Setting?

Gems and minerals with perfect cleavage must be set and worn carefully, since a sharp blow to the stone along a cleavage direction may easily split the gem.

Spodumene is well-known for being difficult to cut. Even topaz offers occasional problems to the cutter who isn't aware of its cleavage direction, because it's virtually impossible to polish a surface that is parallel to a cleavage plane.

Mineral Fracture

Fracture is the way a mineral breaks other than along cleavage directions.

The descriptive terms for this property are: conchoidal, fibrous or splintery, hackly, and uneven.

Conchoidal Fracture

A conchoidal fracture is shell-like and is distinguished by concentric curved lines. This is the way glass breaks.

Fibrous/Splintery Fracture

The terms fibrous and splintery are often used interchangeably to denote a fracture that creates sharp, elongated points in a mineral.

Hackly Fracture

A hackly fracture produces sharp, jagged points.

Uneven Fracture

An uneven fracture produces a rough, uneven surface. Uneven fractures don't have sharp points like splintery and hackly fractures.

Mineral Parting

The term parting refers to breakage of minerals along directions of structural weakness, such as twinning.

However, unlike cleavage, parting isn't present in all minerals. Parting is a result of the individual growth of a particular stone and not the atomic or crystal structure.

Editor's Note: The original article has been updated and adapted for the Mineralogy Certification course.

Joel E. Arem, Ph.D., FGA

Dr. Joel E. Arem has more than 60 years of experience in the world of gems and minerals. After obtaining his Ph.D. in Mineralogy from Harvard University, he has published numerous books that are still among the most widely used references and guidebooks on crystals, gems and minerals in the world.

Co-founder and President of numerous organizations, Dr. Arem has enjoyed a lifelong career in mineralogy and gemology. He has been a Smithsonian scientist and Curator, a consultant to many well-known companies and institutions, and a prolific author and speaker. Although his main activities have been as a gem cutter and dealer, his focus has always been education. joelarem.com

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