How many different birthstones are there?

According to the American Gem Society, there are twelve birthstones, which is one for each month. However, there remains some overlap between "traditional" and "modern" birthstones.

The list of modern birthstones dates from 1912, when the National Association of Jewelers released a list. This list, periodically updated in the intervening years, contains one or more birthstones per month. Many of the traditional and modern birthstones remain the same, but the more recent lists replace relatively obscure stones with stones more commonly in use. An example of this is the month of December, for which bloodstone is the traditional birthstone and turquoise is the modern option.