Because of these magical powers, they were extensively used as talismans and amulets, as predictors of the future, as therapeutic aids, and as essential elements of many religious practices.
The origin of the belief that a special stone was dedicated to each month of the year and that the stone of the month was endowed with a peculiar virtue for those born in that month may be traced back to writings from the first and fifth centuries. The authors of these writings distinctly point to the connection between the twelve stones of the Jewish high priest’s breastplate and the twelve months of the year, as well as the twelve zodiac signs. Later, in the eighth and ninth centuries, the interpreters of the Book of Revelation began to ascribe to each of those stones attributes of the twelve apostles.
"Astral" or "zodiacal" gems were designated as such because they were believed to be peculiarly and mystically related to the twelve signs of the zodiac. "Talismanic" gems were the stones of the twelve guardian angels who were assigned to those born in each month.