Summary
Just as wood is easier to split with the grain than against it, gemstone cleavage is the tendency of certain crystals to break along definite plane surfaces. If there are planes in a crystal structure with relatively weak atomic bonds, the crystal is more likely to break along those planes.
Reading time: 3 min
Fluorite - gemstone cleavage

Green fluorite with prominent cleavage. Photo by Eurico Zimbres and Tom Epaminondas. Licensed under CC By-SA 2.0 BR.

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

Gemstone 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 gemology only applies to crystalline materials. For example, 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.

conchoidal gemstone fracture - gemstone cleavage

Of course, glass can break. However, since it has no crystalline structure, it doesn’t have internal cleavage planes. Glass has an amorphous atomic structure. “The Shape of Broken Glass (2),” photo by zeevveez. Licensed under CC By 2.0.

Cleavage Grades

Gemstone 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

Gems 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.

anhydrite - gemstone cleavage

A very fragile gemstone, anhydrite has perfect cleavage in one crystallographic direction and good (or imperfect) cleavage in another. Anhydrite blades on calcite specimen, © Rob Lavinsky, www.iRocks.com. Used with permission.

Can Gemstone Cleavage Affect Faceting and Jewelry Setting?

Gems 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 gemstone surface that is parallel to a cleavage plane.

spodumene - gemstone cleavage

Spodumene, octagonal Barion-style cut, approximately 13 carats. Photo by ProteusX. Licensed under CC By-SA 3.0.

Gemstone 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.

obsidian conchoidal fracture - gemstone cleavage

Obsidian, a naturally occuring glass, has a conchoidal fracture. Photo by Siim Seep. Licensed under CC By-SA 2.5.

Fibrous/Splintery Fracture

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

vivianite fibrous fractures - gemstone cleavage

This vivianite crystal shows internal fibrous fractures. 3.1 x 1.5 x 1.3 cm, Tomokoni adit, near Canutillos, Colavi, Potosi, Bolivia. © Rob Lavinsky, www.iRocks.com. Used with permission.

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.

Gemstone 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 specimens of a given species. Parting is a result of the individual growth of a particular gemstone and not the atomic or crystal structure of a gem species.

fluorite twinning - gemstone cleavage

All fluorites have perfect cleavage. However, not all fluorites show parting, like this twinned specimen. © Rob Lavinsky, www.iRocks.com. Used with permission.