A big part of the value of platinum is its purity rating. The more platinum in an alloy, the stronger and more valuable the metal will be. On a piece of platinum jewelry, you can generally expect to find a marking identifying how much of the alloy consists of platinum, and which other metals have been alloyed with it.
Platinum is an exceptionally hard and durable metal. As such, it is most often alloyed with other metals in order to make it more malleable for the sake of making jewelry. You can find platinum jewelry with many different levels of purity on the market, but there are only two alloys that are considered high-grade platinum. These are the 950 platinum and the 900 platinum.
The purest platinum you can generally expect to find in your jewelry, 950 platinum gets its name from the fact that it consists of 95% platinum. The other five percent consists of various other metals, usually cobalt, copper, iridium, palladium, rhodium, or tungsten. Pieces with 950 platinum will usually be marked with “PT” or “PLAT”, indicating that the platinum is at least 95% pure.
When a platinum alloy is 90% pure, it is called 900 platinum. The other ten percent of this alloy is usually either iridium or palladium. You can identify a piece of 900 platinum by its marking, which should say “900PLAT”.
Any alloy that consists of less than 90% platinum will be labeled in much the same manner as 900 platinum, with a three-digit number followed by “PLT”, “PLAT”, or “PLATINUM”. The number indicates the number of platinum parts per thousand. For example, a piece marked “585PLAT” has 585 parts of platinum out of one thousand total parts, making for 58.5% platinum.
In the US, any jewelry made with a platinum alloy can be marked as platinum jewelry so long as the alloy is at least 50% platinum and at least 95% “platinum group metals”. The platinum group metals include platinum and the metals that are close to platinum on the periodic table, allowing them to blend well with the metal. These metals include iridium, palladium, ruthenium, rhodium, and osmium. On lower-grade platinum pieces, the ratio of all other metals in the alloy should also be disclosed.