Anatomy of a cut gem

Naming the parts of a stone

A cut gem is not simply a polished stone: it is an optical instrument, in which every part plays a precise role in the path of light. Knowing the vocabulary lets you understand why two stones of the same weight and colour can have radically different brilliance.

The journey of light

The table is the large flat facet on top — the window through which light enters and exits. Its width directly influences perceived brilliance: too small and the stone looks dead; too large and it loses its fire.

The crown is the upper section, between table and girdle. Its angled facets disperse white light into coloured flashes: this is what creates the gem's fire, those rainbow flashes that make a diamond's charm.

The girdle is the thin band running around the stone at its widest point. This is where the jeweller grips and sets it — it protects the rest of the gem from knocks, and its thickness is a criterion of cutting quality.

The pavilion is the whole lower section, invisible once the stone is mounted. Its facets act as internal mirrors: they send light entering through the table back to the observer's eye, by total internal reflection.

The culet, finally, is the point at the very bottom, sometimes cut as a tiny facet to prevent chipping. On antique stones it is often wide enough to be visible through the table.

Why a stone “leaks” light

This is the decisive point, and it explains value: if the pavilion is too deep, light exits through the sides instead of returning to the eye — the stone looks dark at its centre. If it is too shallow, light passes straight through and escapes out of the back: a dull “window” appears in the middle of the stone, and you can literally see through it.

A successful cut is a compromise of angles calculated to a tenth of a degree. That is why two one-carat diamonds of the same colour and clarity can vary in price by a factor of two on the quality of the cut alone.