After the Cataclysm ripped the world asunder, new minerals have appeared. Platinum, a precious metal that was previously to rare to become an official coinage has now appeared in large numbers, along with other metals such as Elementium (Which wouldn't make an ideal currency). Shortages of Gold has lead the noble Alliance and Orcish Horde to adopt it as a new currency. 1 Platinum piece equals approximately 1,000 Gold pieces.
Just an idea I came up with. What do you guys think?