Traditionally, a birthstone is associated with each month of the year. For example, the birthstone for January is a garnet, while 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 idea of birthstones has a place in many traditions, customs, and belief systems.
You can learn more on birthstones at https://www.americangemsociety.org/page/birthstones