Attributing precious and semi-precious stones and to a set of twelve objects can be traced back to the ceremonial breastplate used by Moses' brother, a Jewish High Priest named Aaron. As described in Exodus 28, verses 15-30, the breastplate was adorned with 12 stones, one for each tribe of Israel. Twelve precious stones were also assigned to each of the Apostles in the New Testament.
By the Middle Ages, sets of stones were matched up with the angels and archangels of the Bible and the signs of the zodiac. In 18th century Poland, single stone amulets for each month of the year were made to be worn as a folk remedy for diseases.
The current assignation of stones to months was established by the American National Association of Jewelers in 1912. Today, birthstones are worn for good luck and to enhance the positive properties associated with each gem.
Birthstones by month: