How are birthstones assigned to each month?

Modern birthstones are assigned according to a standardised commercial list made by the Jewelers of America in 1912, which updated and codified older links that trace back to the biblical Breastplate of Aaron, along with ancient astrological signs.

In past times, people dressed themselves in different gemstones each month because of astrological ideas and imagined mystical powers. The 1912 standardisation matched specific stones to calendar months, making jewellery sales easier. This list is sometimes revised by trade groups, like the American Gem Trade Association (AGTA), to bring in new gems, for example, tanzanite, which was added in December 2002.