Gem cutting began with beads and polished dome shapes; faceting came much later, and both survive because they do different jobs. A cabochon — a gem with a smooth, domed top and a flat back — is the older style. A faceted stone carries a pattern of flat polished faces that together act as a machine for returning light to the viewer’s eye.
What each cut does with light
Cabochons show colour, phenomena and interior texture: the star in a star sapphire, the eye of a cat’s-eye chrysoberyl, the banding of agate all live in domes. Faceting is for transparency and brilliance: the cut angles are computed so light entering the top of the stone reflects off the back facets and exits through the front. That is why faceted stones are judged on proportions, and why a badly cut stone looks glassy no matter how fine the rough.
Choosing as a designer
Designers choose cut style on material, meaning and wear. Opaque and phenomenal stones want cabochons; transparent stones usually want facets. Mélée — small faceted stones in calibrated sizes — fills a design with sparkle at controlled cost, and calibrated cabochons of standard diameter drop into standard bezel findings. Drawing both accurately, including the ellipse of a cabochon’s dome and the facet planes of a cut stone, is part of design training at JDMIS (JD100 fundamentals, JD300 illustration); the gem materials themselves are covered in the gem trade course.