Platinum-Iridium is a platinum alloy stiffened with iridium to overcome the softness of the pure metal. Two grades dominate: Pt900/Ir runs 90 percent platinum to 10 percent iridium and reaches roughly 110 HV annealed, while Pt950/Ir uses just 5 percent iridium and sits softer at about 80 HV. For context, pure platinum measures around 56 HV. Americans called the alloy irido-platinum in the 1920s, and through the 1930s to 1950s case backs were stamped IRIDPLAT, IRID PLAT, or 10% IRID PLAT. Iridium has a powerful hardening effect even at low concentrations, which is why it was historically favored for screw-down case backs and springs.
You see this material at its purest in Art Deco diamond watches from American makers such as Hamilton, Wittnauer, and Gruen, most of which read IRIDPLAT on the back. These vintage pieces typically trade at auction for $1,500 to $4,500 on platinum melt value, with diamond-set examples on a running caliber reaching $3,000 to $8,000 and up. Japan used a harder Pt850 grade with 15 percent iridium, around 160 HV, for case backs and springs, stamped Pt850. Most modern Swiss houses have since moved to ruthenium-alloyed Pt950 because it machines and casts more kindly; Rolex, for instance, uses platinum-ruthenium. Genuine Platinum-Iridium is therefore now a more historical, specialist niche.
When buying, read the case back: IRIDPLAT, IRID PLAT, or a Pt900/Ir mark signals a true platinum-iridium alloy, whereas a bare Pt950 stamp usually points to a modern ruthenium case. US FTC rules require 95 percent purity to stamp simply Platinum, so a 90 percent piece must instead be marked 900Pt, giving the platinum content in parts per thousand. This material suits collectors who love Art Deco styling and the dense, matte-white character of platinum, and who want a robust piece that rarely falls below melt value. Older iridium cases do not take quite the mirror polish of softer grades and need correct platinum solder for repair, so a competent casemaker matters.
For pricing context across the broader platinum segment, modern references help: the Patek Philippe Calatrava 6196P in platinum retailed near $47,000 in 2025, A. Lange & Söhne's Saxonia Thin in platinum sits around $45,000 to $48,000, and Audemars Piguet's Royal Oak in 950 platinum starts above $100,000 new. Most of those contemporary pieces use ruthenium-alloyed Pt950, so for true Platinum-Iridium the collector's best hunting ground remains vintage American auctions and estate jewelers.