1. Cut (How Well Cut)
Learn about the cut of a diamond ...
Cut deals with a diamond's proportions and angles, which determine how much it sparkles. Cut is different from shape (such as round, princess, oval, etc.). There are five cut grades, ranging from ideal to poor.
A properly-cut diamond will refract the light that enters it and return it through the top to produce the highly-desired sparkle. If the diamond is too shallow or too deep, some light will escape through the bottom, giving a shadowy appearance when viewed from the top.