The most-touched, least-considered part
The crystal is the transparent cover over the dial, the surface you look through every time you check the time and the part most exposed to the world's knocks. Despite this constant presence it is routinely ignored in discussions of a watch, which is a mistake, because the choice of crystal materially affects a watch's appearance, durability, character, and even its value. Three materials dominate, and each represents a genuinely different set of trade-offs that a buyer should understand.
Acrylic: warm, vintage, forgiving
Acrylic — also called Plexiglass or Hesalite — is essentially a high-grade plastic, and it was the standard crystal until the 1970s. Its disadvantages are obvious: it scratches easily. Its advantages are subtler and genuinely loved. Acrylic has a warm, slightly soft optical quality that flatters vintage dials; it can be formed into the high domes that define mid-century looks; it is shatterproof, flexing under impact rather than cracking; and its scratches are trivially removable — a dab of polishing paste and a cloth buffs them out at home, something no other crystal allows. Acrylic remains in use on watches where it belongs: vintage reissues, and famously the Omega Speedmaster Moonwatch, which keeps its Hesalite crystal partly for heritage and partly because NASA preferred a crystal that would craze rather than shatter into floating shards in a capsule. Acrylic is the connoisseur's choice where character matters more than scratch resistance.
Mineral: the practical middle
Mineral glass is ordinary hardened glass, and it occupies the middle ground — harder to scratch than acrylic, cheaper than sapphire, but inferior to both in its own way. It resists scratches better than acrylic but, once scratched, cannot be easily polished out; and it is more prone to shattering than acrylic while less scratch-resistant than sapphire. Mineral glass is mostly found on budget and mid-range watches as a cost compromise. It is the least characterful of the three — neither the warm vintage charm of acrylic nor the hard clarity of sapphire — and its presence on a watch is usually a sign of where the maker chose to save money.
Sapphire crystal is synthetic corundum — the same material as the gemstone, grown in a lab — and it is second only to diamond in hardness among common materials. This makes it virtually scratchproof in normal use: keys, desks, and door frames leave it unmarked. It has been the standard on serious watches since the 1980s and is now expected on anything above entry level. Its trade-offs are that it is brittle (a sharp impact can crack or shatter it, where acrylic would flex), it is expensive, and untreated it reflects light strongly — which is why good sapphire crystals carry anti-reflective coating, ideally on both sides, to restore the clarity its hardness would otherwise cost. The presence and quality of AR coating is itself a mark of how seriously a watch was made.
Shape, coating, and reading the choice
Beyond material, the crystal's profile shapes a watch's character as much as any dial decision — a high "box" or domed crystal reads vintage and catches light at the edges; a flat crystal reads modern and clean; a slightly domed sapphire splits the difference. Anti-reflective coating determines how clearly you actually see the dial and how the watch photographs. For a collector, the crystal is also a dating and originality clue: an acrylic crystal on a vintage watch is often correct and desirable, while a replacement sapphire on a vintage piece is a modernization that purists may dock. Reading the crystal — its material, its profile, its coating, its correctness for the watch — is part of reading the watch as a whole, and it is information sitting in plain sight, directly in front of the dial everyone else is staring at.
The crystal is a real decision disguised as a piece of glass. Acrylic offers warmth, shatter resistance, and home-polishable scratches at the cost of marking easily; sapphire offers near-perfect scratch resistance at the cost of brittleness and reflection; mineral sits between them as a compromise. Each suits a different watch and a different owner — and learning to read which a watch wears, and why, is one more way the object tells you what it is trying to be.