Synthetic sapphire — hardness 9 on the Mohs scale, approximately 2,000 Vickers — has become the reference material for high-end watch crystals not for aesthetic reasons, but for one concrete property: it resists scratches better than any other glass used in watchmaking, acrylic or mineral. The downside is that it's more brittle to impacts than many believe: concentrated pressure on an edge can shatter it, while an acrylic crystal scratches but doesn't break. For this reason, Panerai, on a 47 mm case, deliberately chose Plexiglas on the front, reserving sapphire for the caseback.
Crystal geometry changes things optically and mechanically. The flat crystal is the least expensive to produce. The domed crystal is convex on one face only. The double dome features two convex curved surfaces, one inward and one outward: it creates a lens effect that increases the perceived depth of the dial and is the typical profile of vintage-inspired watches, such as the Oris Artelier S or the Cuervo y Sobrinos 1935. The Corum Bubble pushes the concept to the extreme, with an 8 mm thickness that produces a noticeable and deliberate optical distortion. To work complex shapes like the cylindrical form of the Rado Anatom, Rado uses diamond tips: at 2,000 Vickers there are no alternatives.
AR treatment — anti-reflective — is a multilayer coating that reduces reflections on the surface. It can be applied to one side or both sides; in some cases, such as the Swiss ARdur®, it is certified for specific sides. On the front crystal it improves readability in backlighting; on the transparent caseback it allows you to observe the movement without light disturbance. Application only inside is a compromise choice: it protects the coating from wear but reduces effectiveness.
Sapphire has found applications beyond the front crystal. The bezel of the Favre-Leuba Deep Blue Revival uses a sapphire insert in place of the original anodized aluminum: the graduated scale doesn't fade. Chopard patented a monobloc sapphire gong for the Strike One: the crystal vibrates directly when struck by the hammer, producing the sound. On the caseback, some manufactures apply decorative metallizations between the crystal and the titanium case, exploiting transparency to create depth effects. The sapphire prisms of the MB&F HM8 Mark 2 project and magnify the hour discs through a double curvature with active optical function. These are uses that my workbench from the Eighties had never seen: the material is the same, but the engineering around it has changed radically.