Platinum is the densest metal used in watchmaking: 21.45 g/cm³ against 19.3 for 24-karat gold. This density is why it works so well in micro-rotors — a small platinum disc accumulates enough rotational inertia to wind the barrel efficiently, keeping the movement thin. I've seen it applied well in Patek Philippe's caliber 2550, Laurent Ferrier's LF270.01, and Czapek's SXH5 and SXH7.1, where the micro-rotor is even in recycled platinum. Tungsten does the same job at much lower costs — similar density, price at 10% — but in the upper segment platinum remains the declared choice.
The standard alloy is 950 platinum: 95% pure platinum, the remaining 5% usually ruthenium, iridium, or cobalt depending on the manufacturer. Vacheron Constantin in the Overseas 2500V ref. uses a 950 alloy with copper and gallium, then thermally hardened, to improve scratch resistance without altering density. Panerai calls their version Platinumtech, still at 95% but with a surface treatment that makes it harder than conventional 950 platinum. These are proprietary alloys: same purity on paper, different behavior at the bench.
Pure platinum is white-gray, it does not oxidize, it does not shift over time. With use it scratches and develops an opaque patina that many collectors appreciate — the metal doesn't wear away, it moves. This characteristic distinguishes it from rhodium-plated white gold, which instead loses its coating and needs retreating.
Recognition markers vary by maison. Cartier engraves a red ruby on the caseback of 950 platinum watches. Patek Philippe sets a small diamond between the lugs. Rolex identifies its platinum models with an ice-blue dial, though in recent years it has introduced exceptions. Rolex also uses the trade name Rolesium for hybrid configurations — Oystersteel case, platinum bezel and caseback — such as the Daytona 126502.
On the dial platinum appears in two ways: as powder in Japanese decorative techniques (in the Credor Goldfeather GBBY967 the raised taka maki-e appliqués are covered with platinum powder) and as galvanic plating on brass bases, with a silvered effect and rippled surface. Platinum galvanizing is more common than it seems: it is not the same material as the case, but produces a neutral and stable color that yellow gold or rhodium do not replicate.
The specific weight of platinum is felt in hand. A 40mm 950 platinum case weighs noticeably more than the same shape in 18-karat gold. For some it is a merit, for others a limitation in daily wearability. I don't have a univocal answer — it depends on the watch and who wears it.