The standard for a well-cut stone, whatever its shape, is the right proportion of its 57 or 58 facets tiny planes cut at angles allowing a maximum amount of light to be reflected through the diamond. The skilled diamond cutter places each of the stone s facets and angles, which act as light-dispersing mirrors, in exact geometric relation to one another. Diamonds cut too shallow or too deep lose light through the side or bottom. All display barely perceptible tinges of color. The professionals grade stones according to their relative colorlessness.
Some rare diamonds have a strong, pure color and are called fancies.
Most diamonds have some inclusions, but they are microscopic and can be seen only under magnification. Diamonds with fewest inclusions are grade I1 or I3.