Cabochon

A cabochon is a stone polished into a smooth dome without facets, usually over a flat or gently rounded base. It is the cut of choice for phenomena. Stars and cat's eyes appear only against a curved surface, so every star sapphire, star ruby and cat's-eye chrysoberyl is a cabochon by necessity, and the shape also suits material whose character reads better in a dome than in sparkle: jade, opal, moonstone, and heavily included corundum carrying strong colour. For star stones, value follows the sharpness and centring of the star and the translucency of the body beneath it. For everything else it follows colour, then how clean the dome looks in daylight.

In practice

The thing to establish at the desk is what the dome is doing. Facet-grade rough is rarely cut this way, since a faceted stone of the same weight almost always fetches more, so a cabochon offered at a faceted price deserves a question. Sometimes the answer is fair: a phenomenal stone, or a colour so strong that facets would only darken it. More often the rough was too included, too fractured or too zoned to facet, and the dome is what was left. Neither is a fault in itself. Both belong on the invoice as a price the stone earns rather than one borrowed from a different cut.

Laboratories record the shape as "cabochon" in the cutting style line and treat the stone as they would any other for identity, treatment and origin. Dealers talk of a high dome or a low dome; a high dome tends to sharpen a star, and it also hides more.

Ask why the stone was cut this way, look at it in daylight while turning it, and check the treatment line before comparing its price with anything faceted. A cabochon is often the honest cut for the material. Occasionally it is the convenient one.