A rough diamond is in some cases remarkable in appearance. Without good cut, even the most beautiful rough diamond that exhibits optimum colour and clarity would lack fire. Therefore, the cut of a diamond is of key importance when considering its quality.

Figure 1. 'Ideal cut' proportions - brilliant round.

Diamond crystals are most often found as octahedrons in their rough state and during medieval times diamonds were worn in this naturally occurring form. This changed with the advent of diamond polishing and cutting. The rough diamonds began to be cleaved, producing two useable crystals. These crystals are usually cut into round brilliants because it is possible to cut two such stones out of one octahedron with minimal loss of weight. If the crystal is malformed or twinned, or if inclusions are present at inopportune locations, the diamond is more likely to receive a fancy cut; any shape other than round. This is especially true in the case of macles, which are flattened twin octahedron crystals.This was publicised by Marcel Tolkowsky in 1914 whose geometric calculations took both the brilliance (the amount of white light being reflected) and fire in the diamond into account, creating the ideal balance between the two.

These calculations now serve as the basis for all brilliant cut modifications and standards. Recent improvements have been made however, to the problem of optimising the scintillation in a round brilliant cut diamond, these gain the maximum life from the stone and lead to the formation of the 'hearts & arrows' effect in brilliant round diamonds.

Polish and symmetry are two important aspects of the cut. The polish describes the smoothness of the diamond's facets and the symmetry refers to alignment of the facets. With poor polish the surface of a facet can be dulled, and may create a blurred or dull sparkle. In the case of poor symmetry, light can be misdirected as it enters or exits the diamond.