found this website hope it answers your question---http://www.tentwothree.com/platinum_jewelry_information.asp
as per the website
900 Plat - 100Irid - means platinum alloyed with 10% iridium.
Many estate jewelry hallmarks will be PT900, indicating that the metal is 900 parts per 1000 of pure platinum. The remaining 100 part are usually made with iridium which makes the jewelry harder.
Dog's Head - French mark indicating platinum used after 1912.
PT950 - platinum. Indicating that 950 parts per 1000 containing platinum. After 1973, The Hallmarking Act was initiated, which will now not allow jewelry to be marked platinum if it falls below 950 parts.
Many fine contemporary vintage platinum rings with precious gemstones such as tanzanite, sapphires, topaz and diamonds now available due to the popularity of the metal.