Each month of the year is associated with a different stone. Carrying the stone associated with your birth month is said to bring good luck and good fortune.
The traditional list is loosely linked to the description of Aaron's breastplate in the bible, which is the oldest written description of a collection of gemstones. The stones in Aaron's breastplate are not well described, and successive translations have muddied the waters further, so no gems can be ascribed to this collection with certainty.
We use the Jeweller's of America list on the stall, but use heliotrope for March and aquamarine as a substitute for diamond which is April's birthstone. There are a lot of different variants to the birthstone list, and if you have learnt a different one, or have always considered your birthstone to be something else entirely -- that's fine too. It's all part of a living tradition.