Determine your birthstone using a list endorsed by the American Gem Society. A birthstone is typically associated with the month in which you were born.
The garnet is the birthstone associated with January births, and the amethyst is associated with February birthdays. The aquamarine is the birthstone for March birthdays. Diamonds are the birthstone for April births, and emeralds are associated with May births. People born in June have two birthstones: pearl and alexandrite. The ruby is July's birthstone, and the peridot is the birthstone for August birthdays. The sapphire is the birthstone for those with September birthdays. Those with October birthdays have two birthstones: tourmaline and opal. Individuals with November birthdays have birthstones citrine and topaz. People with December birthdays have three gemstones to choose from: zircon, tanzanite and turquoise.
The website YourBirthStones.com also mentions several supplemental birthstones. March has the bloodstone as a second birthstone, June has the moonstone as a third birthstone, and the sardonyx is a second birthstone for August.