This is actually a growing industry with Israel and India recently emerging as new centers of skilled diamond cutters. After cleaving the diamond is shaped. Finally the diamond is polished with fine diamond abrasives creating the stone's many angled planes, or facets. Each facet must be in perfect geometric proportion to every other in order to provide maximum reflection. Only when all these stages have been done is the diamond ready to be set and presented to the buying public.
Diamonds have a high refractive index and dispersion.
This is what gives diamonds the brilliance and 'fire' we know. In fact most diamonds are not colorless, but are slightly to noticeably yellow. Diamonds are easier to shop for than other gemstones because they have an accepted grading system that is universal. Diamonds should be faceted so that the maximum amount of light is reflected back through the top of the stone. A poorly cut diamond will appear dull and lifeless.