Diamond Anatomy

Almost every confusing thing on a diamond certificate is really a statement about anatomy. A 61.5% depth, a 57% table, a thin to medium girdle: none of it means anything until you know which part of the stone is being measured.

This page is the vocabulary page. It is worth reading before the cut guide, because cut is entirely a judgement about proportions, and proportions are relationships between the parts below.

The large flat facet on top, and the single biggest facet on the stone. Most of the light that enters a diamond comes in through the table, and most of what comes back out leaves through it too.

Its size is quoted as a percentage of the stone's width, the table percentage. Too large and the diamond looks flat and glassy because there is not enough crown left to split light into colour. Too small and it looks dark. On a round brilliant, somewhere in the mid fifties is the usual sweet spot.

The table is also where an inclusion hurts most, because you are looking straight through it. Our clarity guide covers why position matters as much as grade.

The sloped upper section between the table and the girdle. The crown is where fire is made: light bending through its angled facets separates into colour, which is why a taller crown throws more rainbow flashes and a shallow one looks whiter and flatter.

The Asscher is the clearest example of crown height doing real work. Its unusually tall crown is what produces the stacked, receding reflections people call the hall of mirrors.

The narrow band at the widest point of the stone, separating crown from pavilion. It is the part a setting actually grips, so it matters far more than its size suggests. Prongs sit on the girdle, a bezel wraps it, and a hidden halo tucks underneath it. See setting types for how each one uses it.

Girdle thickness is graded from extremely thin to extremely thick. Extremely thin is fragile and chips. Extremely thick hides weight you paid for in a band nobody sees. Thin to slightly thick is the practical range.

Everything below the girdle, tapering to a point. The pavilion is the engine of the whole stone. Light entering the table strikes the pavilion walls and, if the angles are right, reflects off one side, then the other, and returns up through the table as brilliance.

Get the pavilion angle wrong by a degree or two and that light passes straight out of the bottom instead. That is exactly what a poor cut grade is describing, and it is why a shallow or deep stone looks dull no matter how clean and colourless it is.

The very bottom of the stone, where the pavilion facets meet. On most modern diamonds this is a point and the certificate reads none, which is what you want.

Some stones have a small facet there instead, a leftover from older cutting when a point was liable to chip. A large culet is visible through the table as a grey dot in the centre of the stone, and it is one of the quickest ways to spot an antique or a poorly modernised cut.

A facet is a single polished flat surface. The standard round brilliant has 57, or 58 if it has a culet facet. They are not decorative. Each group has a job.

• Table. 1 facet. The main entry and exit point for light.
• Star facets. 8 triangles sitting against the table, feeding light down into the crown.
• Bezel or kite facets. 8 large four-sided facets on the crown. These do most of the work of splitting white light into colour.
• Upper girdle facets. 16 narrow triangles running down to the girdle.
• Lower girdle facets. 16 more on the pavilion side, controlling the fine flash pattern.
• Pavilion mains. 8 large facets reflecting light back up through the table.

Not every shape uses this arrangement. Step cuts such as the emerald use long parallel facets in terraces instead of triangles, which is why they flash in broad planes rather than sparkling. The shapes guide covers which family each shape belongs to.

Once you know the parts, the numbers on a grading report read as plain English.

• Table percentage. Table width divided by total width. Describes how much of the top is flat.
• Depth percentage. Total height divided by width. This is the number that decides whether a stone looks bigger or smaller than its carat weight suggests. A deep stone hides weight below the girdle where nobody sees it.
• Crown angle. How steeply the crown slopes. Drives fire.
• Pavilion angle. How steeply the pavilion tapers. Drives brilliance, and it is the least forgiving number on the report.
• Girdle thickness. Durability at one end of the scale, wasted weight at the other.
• Symmetry and polish. Whether the facets line up, and how cleanly they were finished. Covered in the symmetry guide.

These are what a cut grade summarises into one word. For round brilliants that summary is reliable. For fancy shapes there is no overall cut grade at all, so you read the proportions yourself.

• It decides sparkle. Pavilion angles return light, crown angles colour it, and the smaller facets break it into flashes.
• It decides apparent size. Two one carat stones can differ by more than half a millimetre across because of depth. See carat weight.
• It decides where inclusions show. Under the table they are exposed; near the girdle they are usually hidden. See clarity.
• It decides how colour reads. Warmth concentrates in the deepest part of the stone and at narrow points, which is why tips on a pear or marquise show tint first. See colour.
• It decides what can hold the stone. Girdle thickness and pavilion depth determine whether a bezel, a prong or a hidden halo will work. See setting types.