Acrylic crystal is the material I've removed most often in thirty years at the bench: polymethylmethacrylate, the same polymer as Plexiglas, machined 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 of the Sixties and Seventies, Omega branded it Hesalite on the Speedmaster. Same material, different trade names.
The property that really distinguishes it isn't scratch resistance—it's poor compared to mineral and nonexistent against sapphire—but its response to impact. Acrylic deforms plastically rather than shattering. On a diver that hits an underwater rock, that's the difference between a scratched crystal and one exploding into the case. It's also why divers in the Sixties could wear a pronounced dome: the material takes 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 transparent caseback, because Omega considered the combination constructively more coherent.
The main flaw 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. Only on acrylic.
Achieving high water resistance with this material is difficult. The Lorier Neptune managed it by declaring 200 meters, but it's an almost isolated case: acrylic is less rigid than glass and behaves differently under pressure, 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 are worth a significant percentage of the price precisely for this reason.
From a patina standpoint, acrylic ages visibly: it becomes slightly hazy, develops fine microlines that raking light highlights. For some collectors that's a flaw. For me it's documentation.