Cut • Color • Clarity • Carat Weight
The cut of a diamond ~ roundness, depth & width, uniformity of the facets determine a diamond's brilliance. Cut is an important characteristic because even if a diamond has perfect color and clarity, a diamond with a poor cut will have dulled brilliance.

The width and depth have the greatest effect on how light travels within the diamond, and how it exits in the form of brilliance.

Too Shallow: Light is lost out the bottom causing the diamond to lose brilliance.
Too Deep: Light escapes out the sides causing the diamond to appear dark and dull.

Anatomy of a Diamond: (click here to view diagram)
Diameter: The width of the diamond as measured through the girdle.
Table: The largest facet of a gemstone.
Crown: The top portion of a diamond extending from the girdle to the table.
Girdle: The narrow band around the widest part of a diamond.
Pavilion: The bottom portion of a diamond, extending from the girdle to the culet.
Culet: The facet at the tip of a gemstone. The preferred culet is not visible with the unaided eye (graded "small" or "none").
Depth: The height of a gemstone measured from the culet to the table.