Just like gold is stamped with "22K" or "18K", platinum has its own set of purity hallmarks. If you look inside the band of a platinum ring, you will likely see a stamp reading PT950 or PLAT. But what do these numbers actually mean for durability and value?
The Parts Per Thousand System
Platinum purity is measured in parts per thousand.
- PT950: The global standard for high-end jewelry. It contains 95% pure platinum and 5% alloy metals.
- PT900: Contains 90% pure platinum and 10% alloys. Common in older jewelry and parts of Asia.
- PT850: Contains 85% pure platinum. This is often the minimum purity legally allowed to be called "Platinum" in many countries without a qualifier.
The Secret of the 5% Alloy
Pure platinum (1000) is too soft to hold diamonds securely. So, jewelers add 5% of another metal to harden it. Which metal they choose changes everything.
1. Platinum-Ruthenium (Pt/Ru)
This is the highest quality alloy used by luxury brands like Cartier and Tiffany. It is incredibly hard, resists scratching better than other alloys, and machines beautifully. If you are buying a plain wedding band, ask if it is a Ruthenium alloy.
2. Platinum-Cobalt (Pt/Co)
Cobalt makes the platinum flow like water when melted. Jewelers use this alloy for intricate castings (rings with lots of tiny details or filigree). It is slightly less hard than Ruthenium but takes a brilliant, mirror-like polish.
3. Platinum-Iridium (Pt/Ir)
This is an older, softer alloy (usually found in PT900). It scratches and dents easily. Modern jewelers try to avoid it for rings, but it is sometimes still used for necklaces where scratching is not a concern.
Always Check the Hallmark
In India, platinum jewelry is governed by the Platinum Guild International (PGI). Look for the Pt950 mark alongside the PGI logo. If a piece simply says "PLAT", ask the jeweler to specify the exact purity before paying.