Round Brilliant

The round brilliant is cut with 57 or 58 facets arranged to send as much light as possible back out through the top of the stone. No other shape returns light as efficiently, which is why a well-cut round looks brighter and livelier than any other diamond of the same weight.

Its modern proportions owe a great deal to the mathematician and diamond cutter Marcel Tolkowsky, who in 1919 worked out the angles that balance brightness with fire, the flashes of colour a diamond throws. Cutting has been refined ever since, but the principle is the same.

The round brilliant is the only shape for which GIA gives an overall cut grade, from Excellent down to Poor. That grade judges proportions, polish and symmetry together, and it matters more than any other line on the report.

Choose Excellent, or the equivalent top grade from another laboratory, and then let colour and clarity take the strain of the budget. A round with a top cut grade and a G colour will outshine one with a weaker cut and a D colour every time.

Because a round brilliant scatters light so well, it hides a little colour and small inclusions better than step cuts such as the emerald or Asscher. In platinum or white gold, G to H colour looks white on the hand. For clarity, an eye-clean VS2 is plenty for most sizes.

The round goes with everything. A four-claw solitaire shows the most diamond; six claws give a softer, rounder outline and extra security. A halo makes it look larger, a hidden halo adds a flash of light from the side, and a bezel gives it a clean, modern edge. Our settings guide covers each one.

With no points or corners, it is also one of the most robust shapes to wear every day.