The most common astrological system for birthstones is similar to the birth month system that you're already familiar with: Not only does it use the same 12 stones, but it also keeps them in order. The thing that sets it apart are the date ranges it uses, which are shifted to align with each Zodiac sign's solar season , rather than the months of the year. For example, January's traditional birthstone is garnet. Under this system, garnet is the birthstone for Capricorn, whose solar season overlaps with the start of January. Skip ahead a few months, and you'll see that October's stone, opal, corresponds with Libra , whose solar season lasts from September 23 to October 22.