The natural property of diamond’s crystalline structure, which results in certain planes having weaker bonding than other planes. This gives them a propensity to split in one or more directions when struck by a blow with the right force. This is simply due to the nature of diamond’s molecular bonds, which are stronger in some directions and weaker in others. This in turn means that diamond can be easily and cleanly “cleaved”, or split, in certain directions during cutting. Sawing is the other method of cutting a diamond, which can be done in more directions than the cleavage planes.
Another use of this term, is when this cleavage plane has resulted in a Fracture in the diamond. Sometimes cleavage and fracture are used in similar ways.