From the time gemstones were discovered, they were believed to have mystical powers and attributes that could be passed to the wearer. The red of ruby was fiery and passionate; cool blue sapphire was calm and composed. About 2,000 years ago, writers began pairing each of the stones and their attributes with the months of the year and the signs of the Zodiac, and with time, the mythology of birthstones evolved.

There are actually many birthstone lists in use today. However, some months still have more than one birthstone (i.e. June - Alexandrite).  Below is the list adopted by the Jewelers of America.