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.
Color is more subjective.
All display barely perceptible tinges of color. The professionals grade stones according to their relative colorlessness. Clarity is, obviously, how clear and free the stone is from naturally occurring inclusions, which can appear as tiny crystals, feathers or clouds and are called the diamond's natural birthmarks. Most diamonds have some inclusions, but they are microscopic and can be seen only under magnifications. Diamonds with fewest inclusions are grade 11 or 13.