The cut of a diamond is the only characteristic which is not natural, but is directly influenced by man. The cut is not just the shape and style of the polished stone it also includes the proportions, symmetry and quality of polish of the finished stone (known as the 'make' in the trade).
It is the cut of the diamond that gives it its sparkle, known as brilliance, and the rainbow of colours known as fire. An equally proportioned, symmetrically cut diamond bounces light around internally from facet to facet, acting like a prism and a mirror, before returning the light back through the top of the diamond.
To achieve this maximum return of light, the pavilion depth, crown angle and table size must all be cut within a range of ideal proportions. If these proportions are not correct, total internal reflection does not occur, and light leaks from the pavilion facets reducing fire and brilliance.
Diamonds are cut in many fancy shapes, but the most popular is the round brilliant cut, with its eightfold symmetry. This cut also makes best use of rough octahedral diamond crystals, with the minimum of waste.