PT950: 95% platinum, 5% other metals. The standard for jewelry.
PT900: 90% platinum. Slightly harder, still excellent.
PT850: 85% platinum. Less common, still valuable.
Higher numbers mean more platinum content. PT950 is the most common and offers the best balance of purity and workability.
Look for the stamp on your chain—"PT950," "950 Plat," or similar markings.