|December||Blue Zircon||Turquoise or Lapis|
Early in this century jewelers got together and created a new list of birthstones. In the modern list you will find only transparent gems. This makes it much easier to create a mothers ring. Imagine if you needed a pearl, a cabochon of turquoise, a diamond and an emerald. This would be an awkward thing to make look nice.
Another interesting point is the addition of alexandrite. This is an incredibly rare gem and not readily available.
The ancient list is still used considerably. Many people mix the two as they feel best. This is acceptable, the modern list has not replaced the ancient one, but supplemented it.
International Gem Society Birthstone Chart
Compliments of The International Gem Society