Cleavage and Fracture Cleavage and fracture are ways gem material will break when an external stress is applied. The ease of breaking will effect the durability, an important attribute of gems. Some minerals have a special way of breaking parallel along planes of atomic weakness, creating smooth flat surfaces. This break is called cleavage. In rough material, a cleavage break may already be obvious or could be observed by breaking the specimen with a hammer.
Fracture is a break in a direction other than along cleavage planes.
A distinctive fracture is called conchoidal or a shell-like break. This break is common in glass, quartz, opal, peridot, and amber. Other fractures are uneven, splintery, granular, even, or subconchoidal. A knowledge of cleavage for the cutter is important as it can lead to an easy first step to the fashioning process for diamonds. The hardness of the mineral refers to its resistance to scratching and abrasion and also to the cutting resistance.