In terms of minerals, the word cleavage explains how a crystal breaks when subject to stress on a particular plane. If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off has no cleavage. One of the characteristics of crystals is that they have cleavage. Cleavage is the tendency of a crystal to break cleanly along distinct planes. Since most gemstones are crystals, cleavage is an issue that gem buyers as well as jewelers and gem cutters need to take into account. Even though a gemstone ranks high on the Moh’s scale for hardness this doesn’t mean that it cannot be damaged. Many stones have cleavage planes within their structures that can split or shatter when subjected to a severe blow at a weak cleavage point.
There are four levels of cleavage: Perfect, Good, Indistinct and None. Gems with perfect cleavage produce a smooth surface along the cleavage plane when it is subjected to a sharp blow. Jewelers use their knowledge of cleavage when cutting precious gemstones in their natural state. Gemstones with perfect cleavage include: Diamonds, Moonstones, Tanzanite and Topaz. Gemstones with no cleavage include: Agate, Amethyst, Citrine, Opal, Ruby and Sapphires. Depending upon the amount of cleavage a gemstone has it will break perfectly or imperfectly.
Minerals cleave along particular crystallographic planes where the atomic bonding is weaker. It is similar to splitting a piece of wood – it splits fairly easily along the grain, but not across the grain. The number of cleavage planes and the angles between them are characteristic of specific minerals. Minerals can have from one to five cleavage planes, and each cleavage plane has a grade or rating, indicating the relative ease with which the crystal can be cleaved. Cleavage is graded as perfect, good or indistinct. Cleavage is often measured by three factors:
1) Quality of Cleavage
2) Number of Sides Exhibiting Cleavage
3) Cleavage Habit
Cleavage was once used to divide large gem crystals. For example, the largest gem-quality diamond ever found, the Cullinan Diamond of 3,1106 carats, was cleaved in 1908 into three large pieces which in turn were cleaved into many smaller pieces. Today in the diamond industry it is more common to use sophisticated saws rather than cleavage to separate crystals.
Cleavage is of special concern to many gem cutters. In gemstones with perfect cleavage, the facets must be cut transverse to the cleavage planes so the gem will be less vulnerable to breakage. Jewelers need to take special care since the heat produced when soldering the setting can cause fissures along cleavage planes and could lead to the gem actually breaking along these fissures. Piercing or drilling should be perpendicular to cleavage surfaces.