900 Platinum — What This Hallmark Means

A "900" mark on platinum jewelry signifies that the piece is made from an alloy containing 90% pure platinum and 10% other metals, typically iridium. This purity standard was the dominant formulation for platinum jewelry from the late 19th century through the mid-20th century, making it especially common in Art Deco, Edwardian, and early Art Nouveau pieces.

The 10% iridium content in 900 platinum provides slightly greater hardness than 950 platinum, which was advantageous for the intricate filigree and milgrain work characteristic of Edwardian and Art Deco jewelry. This higher alloy content made the metal more workable for the delicate, lace-like designs popular during those periods.

While 950 platinum has become the modern standard, 900 platinum remains a perfectly legitimate and valuable purity. Many highly prized antique and vintage pieces carry the 900 mark, and their historical significance often adds considerable value beyond the intrinsic metal worth.

On vintage and antique pieces, the 900 stamp is typically found inside ring bands, on the back of brooches, or stamped on clasps. The mark may appear as "900," "900 PLAT," "PT900," "PLAT 900 IRID," or "900Pt/100Ir" (indicating the iridium content). Some vintage pieces may show just "PLAT" or "PLATINUM" without a numeric purity mark.

Early 20th-century American platinum often shows "IRID PLAT" or "PLAT/IRID" to indicate the iridium-platinum alloy composition. European vintage pieces may carry additional assay marks specific to the country and era of manufacture.

Historical variations include "900 PLAT," "Pt900," "PLAT 900," "900Pt/100Ir," "IRID PLAT," and "90% PLAT 10% IRID." Japanese platinum jewelry frequently uses the Pt900 designation even for contemporary pieces, as this purity remains popular in the Japanese market.

Some pieces from the transitional period between 900 and 950 standards may show "Pt900" alongside other marks indicating the specific alloying metals used. Russian platinum from the Soviet era may carry different markings conforming to the Soviet precious metals marking system.

Some people assume that 900 platinum is inferior to 950 platinum because of its lower purity. In practice, the difference in platinum content is only 5%, and 900 platinum is actually slightly harder and more scratch-resistant due to its higher iridium content. For antique pieces, the 900 designation is period-appropriate and doesn't diminish the piece's value.

Another misconception is that "IRID PLAT" markings indicate the piece is made of iridium rather than platinum. The mark simply indicates a platinum-iridium alloy, with platinum being the dominant metal at 90%.

If you've found a 900 platinum mark, particularly on a piece that appears to be vintage, you may have an antique of considerable value. Consider having it examined by a jeweler or antiques specialist who can assess both its metal content and its historical significance. Art Deco and Edwardian platinum pieces are highly collectible.

Care for 900 platinum the same way as 950 platinum — clean with warm soapy water and store separately from softer metals. If the piece is antique, be cautious about aggressive polishing that might remove the original finish or damage delicate details.