In watchmaking, the term composite doesn't describe a single material but rather an entire construction logic: two or more materials that are naturally incompatible are permanently bonded together to achieve properties that neither possesses alone. From my workbench, I've seen this approach become systematic in the Nineties, and today it's where much of the major manufacturers' technical research is being played out.
The most widespread family is carbon fiber-based. Forged carbon is produced by cutting filaments into short segments, arranging them randomly inside a mold, then compressing everything under heat with epoxy resin. The result is a material roughly five times lighter than stainless steel and significantly more impact-resistant than laminated carbon sheets; the irregular marbled pattern is a direct consequence of the process, not an aesthetic choice added afterward. Variants like Ulysse Nardin and Romain Gauthier's Carbonium use aerospace-grade fibers with specific resins, while Lavoisier Composites' Nylo-Foil replaces part of virgin carbon with recycled nylon from fishing nets and carbon fiber recovered from racing hulls.
The second important family is ceramic-metallic composites. IWC's Ceratanium bonds titanium and ceramic through a sintering process that maintains titanium's lightness while adding ceramic's surface hardness. The Ultra-Cermet used by Parmigiani Fleurier works with different proportions—roughly 40% titanium in a ceramic matrix—and reaches 1450 Vickers hardness. Rado's Ceramos is instead based on titanium carbide at about 90%, achieving metallic luster with technical ceramic scratch resistance.
There are also composites that integrate precious metals: Bulgari's CarbonGold, introduced as early as 1993 and revisited in the Octo Finissimo, combines rose gold with carbon fiber to reduce density while maintaining metal value. Chronoswiss' Titanium Core Nano Shell works in layers instead: a grade 5 titanium core is permanently bonded to a colored composite shell through active bonding, so the color becomes a structural part of the material rather than a finish.
Less frequent but technically interesting is forged quartz fiber: the process is similar to forged carbon, but the quartz base guarantees superior UV resistance and allows deep coloring. Richard Mille's Thin Ply Technology—abbreviated TPT—takes this principle to extremes, laminating ultra-thin layers oriented with precision before polymerization.
A note on HealTech: it's a surface self-healing technology applied to carbon that, when heated between 100 and 150°C, closes micro-imperfections. It comes from the aerospace sector; in watchmaking it's still more of a promise than established practice, at least as far as I've seen to date.