Hesalite is Omega’s name for the acrylic it uses for watch crystals, a kind of acrylic glass made from polymethyl methacrylate (PMMA). It matters because, when hit hard, it cracks rather than breaking into sharp fragments. The Speedmaster has worn it since 1957, and the Speedmaster Moonwatch Professional still does.
Where it comes from
The acrylic behind Hesalite was created in 1933 by the chemist Otto Röhm. Other makers later sold similar acrylics under their own brand names, among them Lucite, Perspex and Altuglas. Hesalite is the name Omega gives to the version it uses for its watch crystals. The material has practical strengths. It costs little to make. It can be shaped in a mould. It can also be polished.
Why did NASA turn down sapphire?
The Speedmaster has had a Hesalite crystal since it was first launched in 1957, before the Moon landing. Omega offered NASA a sapphire crystal for the watch. NASA said no. Its worry was broken glass floating in zero gravity. Hesalite behaves differently from sapphire or mineral glass under a hard blow. It cracks, but it does not break into sharp fragments. It is also elastic, so it absorbs shocks well. Omega still fits Hesalite to the Speedmaster Moonwatch Professional.
How does Hesalite compare with sapphire?
Hesalite is much softer. Its Vickers hardness is about 500, against about 2,000 for sapphire. It has other qualities, though. It does not smear easily, so the crystal does not need frequent wiping to remove fingerprints. Sapphire reflects noticeably more light. On the Speedmaster the Hesalite crystal is domed, with a clear rounded edge. This gives the dial a warmer, vintage look.
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