acrylic crystal

Acrylic crystal is the material I've disassembled most often in thirty years at the bench: polymethyl methacrylate, the same polymer as Plexiglas, worked in thicknesses and profiles calibrated for each case. It's not glass in the chemical sense, but a transparent organic thermoplastic. Rolex called it Tropic in the Submariner models of the Sixties and Seventies; Omega branded it Hesalite on the Speedmaster. The same material, different commercial names.

The property that truly distinguishes it isn't scratch resistance—it's poor compared to mineral crystal and nonexistent compared to sapphire—but its response to impact. Acrylic deforms plastically instead of shattering. On a diver that strikes an underwater rock, this makes the difference between a scratched crystal and one that explodes into the case. It's also why divers in the Sixties could wear a pronounced dome: the material lends itself to heat-forming better than any glass.

The dome, called domed, isn't purely aesthetic. It increases the internal volume of the crystal, reduces oblique reflections, and—optically—produces a slight magnification of the dial that many collectors consider integral to reading a vintage piece. The Hesalite Speedmaster has exactly this effect; the sapphire version doesn't. The Hesalite variant is also the one without a display caseback, because Omega considered the combination constructively more coherent.

The main drawback is well-known: scratches arrive quickly. The good news is they can be removed. Products like PolyWatch or PlexiRevive are abrasives in suspension: you work in circles with a soft cloth until the emulsion is absorbed and the cloth begins to squeak slightly. Never use them on sapphire—too hard—or on treated mineral crystal. Only on acrylic.

Achieving high water resistance with this material is difficult. The Lorier Neptune managed it, claiming 200 meters, but it's an almost isolated case: acrylic is less rigid than glass and works under pressure differently, which complicates gasket sizing. For pieces like the Alpina President, where the crystal shape is atypical, replacement is a real problem: examples in NOS condition command a significant percentage of the price precisely for this reason.

From the perspective of patina, acrylic ages visibly: it becomes slightly opaque, develops fine microlines that grazing light brings into relief. For some collectors it's a flaw. For me, it's documentation.