Synthetic corundum — hardness 9 on the Mohs scale, second only to diamond — became the standard material for quality watch crystals starting in the 1970s, progressively replacing plexiglass and mineral glass. I witnessed this transition firsthand at the Piaget workbench: the first flat sapphires were already a revolution compared to acrylic that would scratch if you looked at it the wrong way. Today the term encompasses a family of geometries and treatments that deserve precise distinction.
The flat crystal is the simplest and least expensive form to produce. The domed crystal — also called domed or dome-shaped — is convex outward: it increases the perceived depth of the dial and revives the aesthetics of vintage acrylics, as in TAG Heuer Carrera's Glassbox construction, where the glass integrates the flange and eliminates the conventional bezel. The double-domed adds convexity also on the inner side: it follows a domed dial beneath more closely and reduces peripheral distortion. The box-domed combines a rectangular profile with central doming, a solution typical of certain square-cased watches.
Sapphire machining is difficult precisely because it's hard: it's milled with diamond-tipped tools, worked slowly, and every additional curve multiplies both the risk of breakage and the cost. A sapphire case with 1.2 mm curvature — like in the Bianchet UltraFino Sapphire — is a mechanically demanding operation disproportionate to the actual extent of the curve.
Anti-reflective (AR) treatment is a thin-layer coating applied by optical interference: it reduces reflected light and increases transmittance. Applied to both surfaces — double AR treatment — it eliminates reflections from both the external and internal surfaces. It's almost always present on quality sapphires; its absence is immediately noticeable when backlit.
There are also more specialized applications. Laser-etched channels on the back of the crystal can contain luminescent material, visually separating luminescence from the dial. Refracting sapphire prisms, like in the MB&F HM8 display system, use refraction to optically reorient information. NanoSapphire treatment enables differentiated chromatic effects within a single element. Transparent sapphire dials — smoky or skeletonized — exploit the material's transmittance to show the movement underneath with a controlled tint. The sapphire case back, called display case back, adds minimal thickness but completely changes how you read the caliber.
One thing I often see overlooked: the optical quality of the crystal also depends on the cleanliness of the edge cutting and the uniformity of thickness. A poorly finished sapphire distorts; a well-made one disappears.