There are actually three birthstones for the month of December. Tanzanite, Zircon and Turquoise are the official birthstones for December. Since Tanzanite was only recently discovered in 1967 it never had the chance to appear on the official gem birthstone list created in 1912. Since it's discovery, Tanzanite has continued to increase in popularity and was added to the birthstone list in October of 2002 by the (AGTA) American Gem Trade Association.
A List of Birthstones by Month