The three materials
Sapphire is synthetic corundum, rated 9 on the Mohs hardness scale — harder than steel, sand or most things a watch encounters. It is expensive to cut and shape, which is why it appears as watches get more expensive.
Mineral glass is hardened silica, rated around 5–6. Seiko's Hardlex is a proprietary variant. It scratches from ordinary contact and cannot be polished.
Acrylic is plastic, rated around 3. It scratches from almost anything, and it also polishes out with a mild abrasive in a few minutes.
The trade-off nobody mentions
Hardness and toughness are different properties. Sapphire resists scratching but is brittle — a sharp impact can crack or shatter it. Acrylic is soft and absorbs impact, which is why it was used in early space programmes: a shattered crystal in a sealed environment is a serious problem, a scratched one is not.
Mineral glass sits in the middle and does neither job particularly well, which is why it is chosen mainly on cost.
Shape matters too
A flat crystal is cheapest. A domed crystal curves the outer edge of the dial and produces the distortion associated with vintage watches. A box crystal — flat on top with vertical sides — creates the most dramatic effect and is the most expensive to produce, which is why the Seiko Presage Cocktail Time is unusual at its price.
Anti-reflective coating, applied to one or both surfaces, does more for legibility than the material choice. A coated mineral crystal is easier to read than uncoated sapphire.
What to actually buy
Under $200, expect mineral and do not treat it as a failure. Between $200 and $500, sapphire is available — the Orient Kamasu and Citizen Tsuyosa both include it — and it is worth prioritising. Above $500, sapphire should be standard, and its absence is a legitimate criticism.
Acrylic is a choice rather than a compromise when the watch is period-correct, as on the Timex Marlin.
