Often confused with the shape and size of the diamond, the cut is the most important factor in choosing a diamond because it is all about the sparkle.
More specifically, the way the diamond has been cut affects how much light passes through the diamond and reflects through the top, giving the diamond its mesmerising brilliance. If the workmanship is poor and cut too shallow, your diamond can appear dull and lifeless; cut too deep and your diamond can appear smaller than it is, also losing its glow.

Ideally, to create dazzling radiance, a well-cut diamond has the perfect balance of proportion, symmetry and polish and this affects the diamond in three ways:
Fire - the rainbow-coloured sparkles that catch your eye in the light
Brilliance - the bright radiant light that shines through the stone
Scintillation - the glittery sparkles and flashes of light when a diamond is in motion

The better the cut, the higher the brilliance your diamond will achieve, so choose a grade that suits the level of sparkle you want in your ring.

Ideal Cut: Showing superior brilliance, these diamonds are very rare and have been precision cut to the most exacting dimensions and proportions. They reflect almost all the light that passes through each facet of the diamond.

Excellent: Radiating magnificent sparkle, these rare diamonds have been cut to provide an exquisite level of fire and brilliance, reflecting nearly all light that enters the diamond.

Very Good: To the naked eye, Very Good cut diamonds reflect nearly as much light as Excellent grade diamonds, providing exceptional brilliance and fire, at a much lower cost.

Good: These diamonds are cut to reflect much of the light that enters, providing premium quality beauty, brilliance and sparkle. Perfect choice to optimise on diamond size.

If you're set on a particular cut grade for your diamond, you can further clarify the quality of that cut by specifying and evaluating the individual components of the cut such the culet, girdle, polish and symmetry. 

 If you're choosing a fancy diamond shape other than round, princess or oval, the cut needs to be individually assessed as the shape affects how the diamond can be cut and are generally less forgiving.