Choose a shape, stretch it and change its depth. The face-up map follows real light paths through the facets to show where the stone is bright, where your own head darkens it, and where light escapes.
Seen face-up from straight above. Each point traces the light back through the stone's facets with diamond's refractive index (2.417). Fire, partial reflections and real-world lighting are left out, so this shows the principle, not the look of a particular stone.
When you look straight down into a diamond, some facets reflect light that would come from directly above you. Your head and shoulders block that light, so those facets look dark. GIA explains the bow-tie this way: part of the light is blocked by the viewer.
A round stone has the same pavilion angle all round. In an oval, pear or marquise the pavilion has to cover a longer distance along the length than across the width, so the facet angles change round the stone. Where they don't send light back to you, the stone looks dark or leaks light.
Cutters balance length, depth and facet angles to keep the bow-tie small. This model does the same in a simple way: it tries several pavilion layouts and keeps the one that returns the most light. Try a long, shallow oval and see the dark areas grow.
Elongated shapes spread their weight over a larger face-up area, so they look bigger than a round of the same carat weight. Sizes below are for a 1.00 ct stone at the depth and table you set, calculated from the model's volume and diamond's density (3.52 g/cm³). The last column is the published trade chart size for comparison.
| Shape | Ratio | Model at 1.00 ct | Face-up area | Looks larger than round by | Trade chart, 1.00 ct |
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Cushions and emerald cuts are usually cut deeper than rounds, which puts more weight below the girdle and makes them smaller per carat than the model shows at the same depth: set the depth to 65–70 % to see this.
Today GIA gives a cut grade only to standard round brilliants. Fancy shapes receive polish and symmetry grades but no overall cut grade. In June 2026 GIA announced cut grades for oval, pear and marquise diamonds from 2027, with details to follow in early 2027.
Until then, the length-to-width ratio, the depth and a careful look at the bow-tie under normal lighting are the buyer's best guides. GIA's published guidance for preferred ratios is shown under the stone above.
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