Onyx is a banded variety of chalcedony (microcrystalline silicon dioxide) with a natural deep-black colour. Watch-grade onyx is cut into thin slabs (typically <1mm thick) for dial use; the slabs are polished to a near-mirror surface. The polishing process must be careful: onyx is brittle and the thin slab can crack under unbalanced clamping. Most onyx dials use natural slab onyx from Brazilian or Indian sources.
Visually, onyx differs from printed-black dials: the natural stone has subtle banding, slightly uneven tone, and a depth of black that printed lacquer cannot match. Light absorbs into the stone and reflects from below the polished surface, producing an 'inky' look. Each onyx dial is slightly different (natural variation in the stone). Used most often on dressy references; Cartier Tank Onyx and Patek Calatrava onyx variants are the canonical luxury examples.
