Christian scholars in the 5th century made the connection between the twelve gems in the Breastplate of Aaron, twelve months of the year, and twelve signs of the zodiac. They theorized that each gem was connected to a certain month or astrological alignment and that they would receive therapeutic benefits for wearing one during that time.
To receive the full benefit, people took to wearing one stone for each month of the year and attributed a different meaning and value to them. Eventually this practice was modified so that a person would only wear the stone for the month they were born in (hence the term birthstone). There was a great amount of disagreement over which stone should represent a calendar month until 1912 when Sears published an “official” list of all the birthstones and the months they represented. Since then there have been a few modifications here and there but the list remains largely unchanged.