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A gemstone which has perfect cleavage is difficult to cut and facet.

This describes the tendency certain gemstones have to break in preferred directions, providing a more or less smooth surface. This is particularly important for the lapidarist, as he or she will need to take this into consideration when deciding on the orientation of the gemstone (where to make the first cut).

In the three-dimensional structure of certain crystals, atoms are bound more tightly to each other in some directions and more loosely in others. As a consequence, when strong forces are applied, relatively clean breaks may occur in these “weakest link” directions. Breaks which are so smooth as to appear as if the gemstone has been  polished are called ‘perfect cleavages’.

Since cleavage, or lack of it, is specific to certain gemstones, it is used by gemmologists as a good identification tool.

Gemstones with easy or perfect cleavage, particularly when they are in multiple directions, are difficult to facet and at greatest risk during the cutting stage. Not all gems have a cleavage, examples being: Tourmalines, Sapphires and Garnets.

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Due to its cleavage Kyanite

is very difficult to facet.