What it is
Sapphire crystal is synthetic sapphire: aluminium oxide (Al₂O₃) crystals grown in a lab via the Verneuil or Czochralski process, then sliced and polished into thin discs. The same material as natural sapphire gemstones, but produced in industrial volumes for watchmaking, optical lenses, and electronics. Watch crystals are typically 1-3mm thick and add 5-15% to a watch's BOM cost over mineral glass.
The hardness comparison
On the Mohs hardness scale, sapphire is 9. Steel is 5-6. Mineral glass (heat-tempered borosilicate) is 6-7. Acrylic (plastic) is 2-3. Diamond is 10. In practice this means sapphire is highly scratch-resistant against everything you'll encounter except diamond rings, sandpaper, and some abrasive minerals. A sapphire crystal will run for 10+ years of daily wear without visible scratches, though it CAN be cracked or shattered by hard impact.
Sapphire vs mineral vs acrylic
Acrylic (plastic): scratches easily but is almost shatterproof; the Omega Speedmaster Moonwatch keeps Hesalite crystal because NASA prefers a crystal that cracks rather than shatters in zero-G. Mineral glass: cheap, moderate scratch resistance, used in mid-tier watches. Sapphire: scratch-resistant but more brittle (impact shatters it). Modern luxury watches default to sapphire; vintage and military uses Hesalite/acrylic.
What to look for
Beyond presence, ask about anti-reflective (AR) coating. Most modern luxury sapphire is AR-coated on the inside (under-coating); some high-end pieces (Rolex, Omega Master Chronometer) are also coated on top. AR coating massively reduces glare and improves dial legibility but can scratch over years of wear. Wiki: Hesalite covers the acrylic alternative.