What is Gemstone Cleavage?

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What is gemstone cleavage? For those of you with overactive imaginations, we can pause a moment so you can regain your composure before we continue.

We’ll start with a brief mental illustration to give you a better idea of gemstone cleavage. Imagine chopping firewood. A log will split much easier along its grain than across it. Now picture a gemstone being split in the same way. A gemstone is easier to split along its grain, or cleavage, just like a piece of wood.

Gemstone Cleavage - The Science

Gemstones have a crystalline structure and most of these structures have a definite plane where the atomic bonds are weaker making the crystal easier to break. These planar surfaces indicate the gemstone’s cleavage. The better the cleavage, the higher the likelihood the gemstone will be smoothly cut along this plane.

In the picture above you see a piece of Mica which has perfect cleavage that runs parallel to the base. If you take a knife and cut it parallel to it’s cleavage plane you will get perfect piece of Mica. If you cut it in the opposite direction it will be much more difficult.

The Grades

Gemstone cleavage is broken into six (6) grades: none, poor, good, distinct, excellent, and perfect. Diamonds, for example, are considered to have perfect cleavage and must be handled with care during the cutting and setting process. Diamonds belong to the cubic system where the raw crystals look like two pyramids stuck together. If you strike a Diamond parallel to this plane it will sheer of perfectly leaving no sharp edges behind.

Gemstones such as rubies and opals, on the other hand, have no cleavage making them more resistant to fracture.

It should be noted that there is a difference between cleavage and a fracture. Cleavage can be predicted as it occurs along a definitive plane of a gemstone. Cleavage can be repeated depending on the number and direction of these planar surfaces. Fractures are very different. They are random and arbitrary and cannot be repeated. These breaks happen anywhere on the mineral and in any direction.