The idea of birthstones -- a gemstone assigned to each month of the year -- is thought to be an ancient one, and scholars trace it back to the Breastplate of Aaron described in the Bible in the book of Exodus. The Breastplate was a religious garment set with twelve gemstones that represented the twelve tribes of Israel. The gems were set in four rows of three: sardius, topaz and carbuncle; emerald, sapphire and diamond; ligure, agate and amethyst; and beryl, onyx and jasper.
So how did we get from gemstones representing the twelve tribes of Israel to birthstones? The writings of Flavius Josephus (1st century AD) and St. Jerome (fifth century AD) made the connection between the 12 stones in the Breastplate and the 12 signs of the zodiac. The idea was proposed that each of the gemstones had special powers associated with the corresponding astrological sign, and that wearing these stones at the right time would have therapeutic or talismanic benefits.
The idea of each person always wearing a gemstone corresponding to the month of his or her birth is a distinctly modern idea that scholars trace to 18th century Poland, with the arrival of Jewish gem traders in the region. But the modern list of birthstones was not defined until 1912, by the National Association of Jewelers (Jewelers of America) in the USA.
The modern birthstone list has been unchanged since 1912. Only recently have we seen two attempts to change it, both curiously associated with the gemstone tanzanite. In 2002 the American Gem Trade Association (AGTA) announced that they had added tanzanite as a birthstone for December, though December already had two birthstones (turquoise and zircon).