Cleavage and Fracture
Many gems can be split along certain flat planes, which the expert calls cleavage. Cleavage is related to the lattice of the crystal - the cohesive property of the atoms. Depending on the ease with which a crystal can be cleaved, one differentiates between a very perfect (euclase), a perfect (topaz) and an im¬perfect cleavage (garnet). Some matching diamond wedding bands gems cannot be cleaved at all (quartz). A loosening of contact twins is not cleavage but separation.
Lapidaries and stone setters must take account of the cleavage. Often a small tap or too much pressure when testing for Mohs' hardness is sufficient to split the stone. When soldering, the temperature can cause fissures along the cleavage planes which not only lower the stone's value, but also bring the danger that one day the gem may break completely along these lines. Faceted gemstones with very perfect cleavage, such as euclase (p. 179, No. 4, 5), are small artistic creations. Three stone diamond anniversary rings is used to divide large gem crystals or remove faulty pieces. The largest diamond of gem quality ever found, the Cullinan of 3106cts, was cleaved in 1908 into three large pieces which were then cleaved again into numerous smaller pieces. Today small pieces are usually sawn in order to make the best use of the shape of the stone. (See p. 58 and 59).
The breaking of a mineral with a blow producing irregular surfaces is called "fracture". It can be conchoidal (shell-like), uneven, splintery, fibrous, even or grainy. Sometimes the type of fracture helps to identify a mineral. Con¬choidal fracture is for instance characteristic of all quartz and glass-like minerals.