The first century Jewish historian Josephus believed there was a connection between the twelve stones in Aaron’s breastplate (signifying the tribes of Israel, as described in the Book of Exodus), the twelve months of the year, and the twelve signs of the zodiac.
Translations and interpretations of the passage in Exodus regarding the breastplate have varied widely, with Josephus himself giving two different lists for the twelve stones. St. Jerome, referencing Josephus, said the Foundation Stones of the New Jerusalem (Revelation 21:19–20) would be appropriate for Christians to use.
In the eighth and ninth century, religious treatises associating a particular stone with an apostle were written, so that “their name would be inscribed on the Foundation Stones, and his virtue. Practice became to keep twelve stones and wear one a month. The custom of wearing a single birthstone is only a few centuries old, though modern authorities differ on dates.
In 1912, in an effort to standardize birthstones, the (American) National Association of Jewelers (now called Jewelers of America) met in Kansas and officially adopted a list.
The Jewelry Industry Council of America updated the list in 1952 by adding Alexandrite for June, citrine for November and pink tourmaline for October. They also replaced December’s lapis with zircon and switched the primary/alternative gems for March. The American Gem Trade Association added tanzanite as a December birthstone in 2002.