Cut
The "Cut" of a diamond refers to how well its facets interact with light. The diamond's "brilliance" depends on the precision of its cut. When a diamond is cut with perfect proportions, light can reflect internally and disperse through the top, creating that sparkling effect. An excellently cut diamond is characterized by:
Brightness: The white light reflected from a diamond.
Fire: The breaking up of white light into all the colours of the rainbow.
Scintillation: The amount of sparkle a diamond produces and the pattern of light and dark areas caused by reflections within the diamond.