Sapphire vs mineral crystal

Why sapphire is the default recommendation

Because the failure mode it prevents is the one that actually happens. Watches get dragged along door frames, desks and car doors constantly, and every one of those contacts is a light abrasion. Over ten years a mineral crystal accumulates hundreds of them and starts to haze; a sapphire one does not.

The counter-risk — that sapphire is more brittle and can chip on a hard impact — is real but much rarer. A blow hard enough to chip sapphire is usually hard enough to have been memorable, whereas the abrasion that ruins a mineral crystal happens without you noticing.

The case for acrylic, which nobody makes

Acrylic is treated as the cheap option, and on a five-year horizon it is. On a forty-year one it is arguably the best material here, for a reason that has nothing to do with hardness: it is the only crystal an owner can restore themselves.

A scratched acrylic crystal comes back to clear with a cloth and a few dollars of polishing paste, in about ten minutes, as many times as you like. A scratched sapphire crystal cannot be polished at all — the material is harder than anything you would use on it — so the only fix is replacement at a workshop. For somebody who intends to maintain a watch personally for decades, that flips the calculation entirely.

It is also the most shatter-resistant of the four by a distance, which is why acrylic was the standard on tool watches for most of the twentieth century.

Hardlex, and what Seiko is doing

Hardlex is Seiko's own hardened mineral glass, fitted across the 5 Sports line and on the Prospex Turtle. It is harder than plain mineral and clearly softer than sapphire, and like mineral it cannot be polished.

It is the single clearest place Seiko saves money at the entry level, and it is why the Orient Kamasu — sapphire at a similar price — scores higher on this site than the Seikos it competes with. On a Seiko you intend to keep, a sapphire replacement at service time is a common and reasonably priced job, which is the mitigation.

What about anti-reflective coating?

Worth having, and worth knowing where it is. Coating on the inner surface only — which is what Seiko specifies on the SPB317 and SPB453, and Tissot on the PRX — is durable, because the coating is protected by the crystal above it. Coating on the outer surface reflects less but wears off in patches over years, and looks worse than no coating at all once it starts to go.