Why Rolex switched to ceramic
Pre-2005 Rolex sport watches used anodised aluminium bezel inserts with painted numerals. Aluminium is soft (scratches easily), the anodised colour fades with UV exposure (vintage Submariner bezels famously fade to grey or 'ghost' patina), and the painted numerals wear off. Cerachrom solved all three: hard ceramic doesn't scratch in normal wear, infused colour doesn't fade, and the metallic-PVD numerals are protected inside the ceramic surface.
How it's made
Rolex sinters zirconium dioxide powder at high temperature into the bezel shape, then mills the numeral grooves into the ceramic. Platinum or gold PVD (physical vapour deposition) is then applied: a thin layer of metal vapour bonds chemically to the ceramic, filling the grooves with metallic numerals that are integral to the bezel material. The bezel is then polished to a mirror finish. The process is proprietary and time-consuming; Cerachrom production is a meaningful contributor to Rolex's bezel cost.
What you see
Submariner 126610LN (black bezel): black ceramic, platinum PVD numerals. Submariner 126610LV ('Kermit'): green ceramic, platinum PVD numerals. GMT-Master II 126710BLNR ('Batman'): two-tone bezel made from a single ceramic ring split black/blue (a Rolex manufacturing achievement). GMT-Master II 126710BLRO ('Pepsi'): black/red bicolour ceramic. Daytona ceramic bezel: black or green ceramic with tachymetric scale.
How it ages
Modern Cerachrom is essentially unchanged after 10+ years of daily wear. It doesn't fade, doesn't scratch in normal contact (only diamond-grade abrasives or hard impact), and the numerals don't wear off. Vintage aluminium bezels, by contrast, often need replacement after 10-20 years; Cerachrom bezels are expected to outlive the watch movement. The trade-off: ceramic is brittle to hard impact (a sharp blow can chip or shatter the bezel), where aluminium is dent-resistant.