Sapphire Crystal

Synthetic corundum — 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 a single concrete property: it resists scratches better than any other glass used in watchmaking, whether acrylic or mineral. The downside is that it is more brittle to impacts than many believe: concentrated pressure on an edge can shatter it, while an acrylic crystal scratches but does not break. For this reason, Panerai, on a 47 mm case, deliberately chose Plexiglas for the front, reserving sapphire for the caseback.

The geometry of the crystal changes things optically and mechanically. The flat crystal is the least expensive to produce. The domed crystal is convex on a single face. The double dome presents two convex curved surfaces, one facing 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, as in the Oris Artelier S models or the 1935 Cuervo y Sobrinos. The Corum Bubble pushes the concept to the extreme, with a thickness of 8 mm that produces obvious and deliberate optical distortion. To work complex shapes like the cylindrical geometry of the Rado Anatom, Rado uses diamond-tipped tools: at 2,000 Vickers there are no alternatives.

AR treatment — anti-reflective — is a multi-layer coating that reduces reflections on the surface. It can be applied on one side or both; in some cases, like the Swiss ARdur®, it is certified for specific sides. On the front crystal it improves readability in backlighting; on the transparent caseback it allows observation of the movement without light interference. Application only on the 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 does not fade. Chopard patented a one-piece 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 1980s workbench had never seen: the material is the same, but the engineering around it has changed radically.