It has been a long standing tradition that every month of the year has a birthstone associated to it and your stone is determined by the month in which you are born.

Where this tradition originates from stems back to the sacred breastplate of the Priest Aaron that is described in the book of Exodus in the Bible.  This religious garment was set with twelve gemstones that represented the twelve tribes of Israel.  From here scholars in both the 1st Century AD and 5th Century AD made a connection between the 12 stones in the breastplate and the 12 signs of the zodiac.  However the idea of someone wearing a gemstone that corresponds with the month of their birth is fairly modern and can be traced back to Poland in the 18th Century.

 

Chart of Birthstones

 

The modern list of gemstones was not defined until 1912 when the National Association of Jewelers in the USA created the list that we know today.  This list remained unchanged until 2002 when the American Gem Trade Association (AGTA) announced that, due to its significance as a rare and precious gemstone, they had added Tanzanite as a birthstone for December (joining Ziron and Turquoise).