The bezel is the ring that runs around the crystal, either fixed or rotating, and determines a substantial part of a watch's functionality and geometry. It's not ornament: it's structure. The distinction that really matters, on the workbench, is between fixed and rotating bezel, and within the latter between unidirectional and bidirectional.
The fixed bezel doesn't move. It may carry an engraved or printed tachymeter scale, or be smooth or knurled like on the Datejust: in that case the choice is tactile and aesthetic, not functional. On the Milgauss 6541, the fixed six-section bezel served as a reading scale for scientific intervals, but its practical utility was already questionable back then.
The unidirectional bezel is the standard on professional diving watches. It rotates only counterclockwise for a precise reason: if accidentally struck, the reference mark shifts toward shorter remaining times, never longer. An error reduces the measured time, not extends it. This is the safety principle. Some manufacturers add mechanical locking systems — the so-called aqua-lock — to prevent any involuntary rotation during immersion. The number of clicks per rotation varies: 60 is the ISO standard, 120 clicks as on the Nezumi Baleine give 3° increments for finer adjustments.
The bidirectional bezel rotates in both directions and is typical of GMT and flieger watches: it serves to read a second time zone on a 12 or 24-hour scale. The Fortis F-39 and F-41 use 24 clicks — one every 15 minutes — with DLC coating for resistance and readability in difficult conditions. The Rolex GMT-Master II also mounts a bidirectional bezel, with a bicolor ceramic Cerachrom insert.
The material of the insert changes everything. Rolex's Cerachrom ceramic doesn't lose color under UV rays and resists scratches. Anodized aluminum is more economical but scratches. The Kurono Diver uses Extra Super Duralumin, an alloy with high resistance to salt corrosion, to keep weight down. DLC applied over steel provides hardness without added mass.
There are also internal bezels, commanded by a secondary crown: Seiko's Alpinist and Longines' Legend Diver are classic examples. They protect the scale from external pressure and prevent accidental rotation during immersion. The technique of laser-cut channels on the back of the sapphire crystal, as in the Ming 37.06, allows instead to contain luminescent material in the bezel without bringing it onto the dial: it's a recent solution, still niche.