History of Birthstones
This tradition of the birthstone goes back to the Book of Exodus, in which the first high priest of the Israelites wore a breastplate with twelve stones, believed to have magical powers, representing the twelve tribes of Israel. It became a tradition for Christians to wear all twelve stones, recalling the breastplate. As time went on, people started rotating through their collection and wearing one stone on its own, based on which one was believed to have heightened powers at that time. This eventually evolved to associating particular gemstones with birth months.
In 1912, a list of birthstones based on the month was standardized in the United States by the National Association of Jewelers (now the Jewelers of America). They were chosen based on tradition, as well as which stones could realistically be sold by American jewelers. The official list has been updated, so you might see different birthstones for your month depending on where you look.