It is the cut that allows a diamond to make the best use
of light. A diamond is cut into many facets at optimum angles
to produce maximum brilliance (sparkle), and light is refracted
internally from one facet to another before being dispersed
thorough the top of the stone.
If the cut is too deep, some light will escape through
the opposite site of the pavilion; too shallow and light
will escape through the pavilion before it can be refracted.
A master cutter ensures full light dispersion, maximising
the inherent beauty of the stone.
The quality of the cut is known as the "make".
If cut properly a diamond reflects the light back up through
the center of the stone to give its characteristic sparkle.