Round Brilliant
The standard round cut with 57 or 58 facets, engineered to return the maximum light. The most common cut there is.
Its proportions were worked out mathematically in 1919 and have barely changed since.
It is the most wasteful cut, discarding more of the rough crystal than any other, which is part of why round stones cost more per carat.
It is also the most forgiving. A round brilliant hides colour tints and small inclusions better than any step cut.
When you are buying
If you want maximum sparkle and are not sure about a stone's colour grade, round brilliant is the safest cut. It is doing the most work to flatter the stone.
Cut
How a stone is shaped and faceted. The one thing that most affects how it looks, and the one buyers examine least.
Facet
One flat polished surface on a cut stone. Their number and angles decide how it returns light.
Princess Cut
A square brilliant cut. Nearly as much sparkle as a round, with far less of the rough crystal wasted.
Diamond
Crystallised carbon, the hardest natural material. Heera in Hindi, and graded on the four Cs.
Cushion Cut
A square or rectangular cut with rounded corners, like a pillow. Softer and warmer in character than a princess.