Watch rubber isn't just one material: it's a family, and lumping them all together is the most common mistake I hear at the bench. I distinguish at least three categories that really matter in daily practice: vulcanized natural rubber, synthetic fluoroelastomer (FKM), and proprietary compounds like Rolex's Oysterflex, which adds an internal stamped metal core to give structure without sacrificing compliance.
FKM — fluorocarbon elastomer class — is today the reference material for serious sports contexts. It resists chemicals, salt water, fuels, and prolonged UV exposure better than any natural rubber. It doesn't attract dust like silicone does, remains flexible even at low temperatures, and holds its shape over time. Independent houses like Baltic use it, as do larger brands, often pairing it with finishes that mimic technical fabrics or textured surfaces.
The term Tropic deserves historical clarification. It originated from Zodiac's production line in the 1960s: a rubber with transverse ribs, sometimes described as "wavy" or "scale-like," designed to resist seawater and sweat in hot climates. Ventilation was the technical goal: the ribs reduced skin contact surface, limiting heat and moisture buildup. From that point on, "tropic-style" became an industry-wide generic term for any rubber strap with that geometry, regardless of manufacturer. You find it today in FKM, in rubber, and in mixed compounds. On vintage divers — the Speedmaster Professional included — the Tropic variant was original equipment, an alternative to the steel bracelet for water use.
Surface textures serve a technical function beyond aesthetics. The hobnail pattern, with regular studs, improves grip and reduces slipping on the wrist. Sailcloth texture mimics sail weave and adds tactile roughness. Both also increase evaporation surface area, which in summer isn't a trivial detail.
Rally-style holes — supplementary closely-spaced holes along the lug — often appear on Tropic FKM models: they allow intermediate adjustments compared to standard holes, useful for wrists that vary with hydration or temperature. A practical solution born in motorsport that transferred to divers and wrist sports without losing utility.