For mechanical-watch enthusiasts who obsess over a movement’s intricately layered architecture of gears, wheels, and bridges, a transparent sapphire-crystal case is the ultimate frame for a compelling kinetic sculpture.
Today’s watches are generally topped with a scratch-resistant sapphire crystal, and many are fitted with a sapphire case back for a peek into the mechanism. But some brands are crafting entire cases out of the transparent material, so you can see the mechanics from all sides and angles.
Synthetic sapphire crystal is made by crystalizing pure aluminum oxide at extreme temperatures to achieve a material chemically identical to natural sapphire gems. The material measures a 9 on the Mohs scale for hardness, compared with diamond, at 10.
Due to its extreme hardness, sapphire crystal is very difficult to machine and polish, requiring diamond-cutting tools. Producing a full case is costly and therefore often reserved for highly technical complications and limited editions. In addition, sapphire weighs less than titanium, so even voluminous cases can remain light.