Cleavage

Cleavage: the breaking or separation of crystalline material along well-defined crystallographic directions, parallel to atomic planes where the atomic bonds between planes are weaker. This results in the formation of flat, smooth, and reflective surfaces that are distinctive enough to serve as diagnostic features for certain species (e.g., Diamond, Topaz, Iolite, Scapolite, etc.) when visible. Not all gemstones exhibit cleavage, of course.

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