You can choose your
birthstone according to the month in which you were born, or according to the zodiac sign. In some cases, you may get the same birtstone either way. But mostly, it is a different gemstone for the month and the zodiac sign. So which should you choose?
The breastplate of Aaron, the origin of birthstone list, has birthstone gemstones corresponding to the zodiac signs and not calendar months. In fact, the present calendar months system was not known at the time of the exodus. The choice of
birthstone gems for each zodiac sign is again open to interpretation. E.g., in the olden days, both rubies and garnets were classified as carbuncles.
So historically, the zodiac signs should be correct. However, birthstone jewelry according to calendar months is the popular choice now.