In watchmaking, crystal is the glass that protects the dial. It's not a generic synonym: it designates both the material and the geometry of that protection, and from my bench I can see that the builder's choices on these two aspects radically change the reading experience and durability over time.
The three main materials are acrylic crystal (plexiglass), tempered mineral, and synthetic sapphire. Acrylic was the standard until the 1970s: it scratches easily but polishes well, absorbs impacts better than the others, and that soft curvature of vintage 1960s-70s watches is nearly impossible to replicate in sapphire without prohibitive costs. Tempered mineral is today's economic compromise: harder than acrylic, softer than sapphire. Synthetic sapphire—artificially grown corundum—is the modern reference: 9 on the Mohs hardness scale, virtually unscratchable in daily life, but more fragile to point impacts than one might think.
Geometry matters as much as material. A flat crystal is simple to produce and replace. A domed crystal—curved outward—acts as a light convex lens, gives optical depth to the dial and evokes vintage aesthetics. The double-domed carries curvature on both faces: more pronounced lens effect, reflections that vary with viewing angle, more complex production. The box crystal or glassbox pushes the dome upward with squared profile, recalling twentieth-century acrylic crystal aesthetics; rare on the caseback, where it adds measurable thickness. The dual-curved curves on two orthogonal axes, useful on tonneau cases and curved lugs.
Anti-reflective (AR) treatment is an extremely thin interference coating. Applied only on the inside it protects the coating from external wear but leaves reflection residue on the outer face; applied on both sides it maximizes transparency but deteriorates sooner. On a heavily domed crystal, reflections change with angle: no AR eliminates them entirely, and anyone telling you otherwise is selling you marketing.
The transparent sapphire caseback—sapphire caseback—is technically the same material as the front crystal, but the stresses are different: it doesn't need to resist the impacts of an active wrist, but must withstand the pressure of the water-resistance seal. On some contemporary pieces like the MB&F HM8 or skeletonized Big Bang, sapphire becomes the optical structure of the dial itself, not merely protection. This is proper material use, not decorative.
A practical note: signed crystals—with the maker's mark engraved or printed—I find regularly on vintage Roamer watches and other minor Swiss brands from the 1950s-60s. They're an authenticity marker worth verifying before any restoration.