The Vickers scale doesn't measure how hard a material is in absolute terms: it measures a surface's resistance to penetration by a diamond pyramid indenter under controlled load. The result, expressed in HV (Hardness Vickers), is the reference parameter in modern watchmaking whenever we talk about scratch resistance of cases, bezels, and bracelets.
The baseline values to keep in mind: raw 316L steel sits around 200–220 HV. This alone explains why a polished steel Submariner or Datejust scratches relatively easily. Grade 2 titanium is even more vulnerable, at roughly 145 HV — soft as butter if untreated. Grade 5, used more often in sport cases, reaches about 320 HV: better, but not enough for someone working in the field.
Surface treatments exist precisely to bridge this gap. DLC (Diamond-Like Carbon) brings the surface to values between 1,200 and 4,500 HV depending on the process: the 4,500 HV that certain manufacturers cite for their coatings represent roughly twenty times the hardness of bare steel. Multi-stage diffusion carbon — as in Hanhart's HD12 steel — reaches 1,200 HV while maintaining a ductile internal structure, avoiding the brittleness typical of very hard materials. Zirconium oxide ceramic (ZrO2) is worth approximately 1,300 HV throughout its mass, not just at the surface: this is the difference between a coating and an integral ceramic material.
One fact I see misunderstood often: high Vickers hardness does not equal indestructibility. A 4,500 HV DLC resists scratches from daily contact, but is more brittle to cutting impacts compared to untreated, tough steel. Very hard coatings tend to chip, not scratch. Ceramic, similarly, is extremely hard but fractures if struck at an angle on an edge. The other aspect that press releases keep silent about: certain high-hardness treatments prevent natural patina. Steel or titanium with a 1,200 HV coating doesn't age, doesn't develop the surface history many collectors appreciate. It's a choice, not a net advantage.
For reference: below 500 HV, the surface scratches with normal wear. Between 800 and 1,200 HV, daily resistance is good. Above 1,200 HV you enter the territory of serious tool watches. The Seiko Diashield, which I often cite as an accessible practical reference, stands at around 500 HV — already significantly better than standard 316L, without the brittleness compromises of more extreme DLC.