Traditionally, a birthstone is associated with each month of the year. For example, the birthstone for January is a garnet, while lucky babies born in April get a diamond as their birthstone.
The origin of birthstones is believed to date back to the breastplate of Aaron which contained twelve gemstones representing the twelve tribes of Israel. The current list dates back to 1912 with only one addition since then the tanzanite was added to December.