The synthetic sapphire used in watchmaking has nothing to do with the blue precious stone: it is artificial corundum, aluminum oxide (Al₂O₃) grown in the laboratory through the Verneuil or Czochralski process, brought to hardness 9 on the Mohs scale. Only diamond surpasses it. I've seen 1970s watches return to the bench with mineral glass reduced to a plowed field; the sapphire crystal, over the same lifetime, remains almost intact. This is why it has replaced tempered mineral in the mid-to-high segment since the early 1980s.
Shape conditions everything. The flat crystal is the simplest to produce and the most common, but it is not optically neutral: it creates flat reflections that overlap the dial. The domed — curved outward — reduces this effect because it disperses light in different directions, but introduces peripheral distortion. The double-domed, convex on both the outside and inside, tends to better follow the geometry of dials that are also curved, and you encounter it on many cushion or barrel cases where surfaces must dialogue. The box domed adds a rectangular element: the central section is flat or nearly so, the edges curve downward, a solution that adapts well to vintage-inspired cases. TAG Heuer's Glassbox — returned to the Carrera in 2023 — takes this concept to the extreme: the crystal extends to encompass the inclined flange, eliminating the traditional bezel and restoring the three-dimensional depth of acrylic crystals from the 1960s.
Anti-reflective (AR) coating is a layer of extremely thin metal oxides deposited under vacuum. Applied to one side only it reduces reflections by about 50%; on both sides it reaches close to 99% light transmission. The crystal becomes almost invisible. The limitation is the delicacy of the coating: aggressive solvents and abrasive cleaning degrade it. I often see it already hazy on watches just a few years old, simply poorly cleaned.
Sapphire is worked only with diamond wheels. The more complex the shape — curved surfaces, asymmetrical profiles, internal grooves to hold luminescence as in the Ming 37.06 — the more the machining cost rises non-linearly. The Bianchet UltraFino with sapphire case at 1.2 mm curvature is a limiting case: milling a curve in corundum requires tolerances that very few manufacturers can manage. The sapphire caseback, now common even on mid-range pieces, adds minimal thickness (Nomos quantifies it at 0.1 mm) but exposes the movement to visibility that demands proper finishing on everything visible from the outside.
Transparent sapphire dials — fumé, smoked, skeletonized or overlaid in sandwich construction — use the crystal as a true constructive element, not merely as a cover. Here the material works twice: it protects and at the same time it shows.