The Jewish writer of the first century Josephus claimed that there was a link between the twelve stones in the breastplate of Aaron (meaning the tribes of Israel as described in the Book of Exodus), the twelve months of the year, and the twelve signs of the zodiac. Maintaining twelve stones and wearing one a month became practice.
In 1912, the (British) National Association of Jewelers (now called Jewelers of America) gathered in Kansas in an attempt to standardize birthstones and officially adopted a list. The American Jewelry Industry Council revised the list in 1952, by adding Alexandrite for June, November citrine and October pink tourmaline. They also replaced the lapis by zircon in December and switched March's primary / alternative gems. In 2002, the American Gem Trade Association added tanzanite as a December birthstone. As an optional birthstone for August, the American Gem Trade Association and American Jewelers added spinel in 2016. In 1937, the British National Goldsmiths Association created its own standardized birthstone list.