In watchmaking, hardness is not an absolute property of the raw material: it is almost always the result of a surface treatment applied over a softer substrate. Understanding this distinction makes a difference when choosing a watch for intensive daily use.
The two scales you will encounter most often are Mohs and Vickers. The Mohs scale is ordinal and comparative: it ranges from 1 (talc) to 10 (diamond) and measures relative scratch resistance between minerals. Common mineral glass sits around 5, synthetic sapphire crystal (corundum, aluminum oxide) reaches 9 and can only be scratched by boron nitride or diamond itself. Acrylic stops at 3-4, it scratches easily but polishes just as easily with products like PolyWatch. The Vickers scale, on the other hand, is quantitative: it measures penetration resistance on metals and coatings, and is the one manufacturers cite in technical datasheets. 316L stainless steel hovers around 150-200 HV in its untreated version.
Coatings change everything. DLC (Diamond-Like Carbon) is amorphous carbon deposited in vapor phase: in the Dianoir version of some manufacturers it reaches 4,500 HV, about twenty times bare steel. PVD (Physical Vapor Deposition) is a broader category that also includes titanium carbide, used to give black finish and additional protection to base titanium. The UltraHex coating on some titanium cases claims 1,200 HV, eight times 316L steel. Citizen's Super Titanium, which combines titanium with the Duratect process, reaches 1,700-2,300 HV while maintaining titanium's weight advantage. Formex brings grade 2 titanium from 145 HV native to 900 HV with specific hardening.
Solid materials with high hardness exist without coating though. Technical ceramic in zirconium oxide (ZrO2) has an intrinsic hardness of about 1,300 HV and requires diamond tools to be machined. The tungsten carbide used by Bvlgari in the platinum bezel of the Octo Finissimo Ultra is a different case: here extreme hardness serves structural rigidity at minimal thicknesses, not just surface resistance. Chopard's Lucent Steel is a proprietary alloy claimed to be 50% harder than conventional steel, while remaining steel in every respect.
A detail I often notice at the counter: high hardness and high brittleness often go together. Ceramic and tungsten carbide resist scratches extraordinarily, but a sharp blow on an edge can chip them. Untreated steel scratches sooner, but absorbs impacts better. It is a compromise that each manufacturer resolves in their own way, and one worth understanding before choosing.