Birthstones have their roots in ancient astrology, and there have been many birthstone lists used over the years. The most common one today is based on a list first publicized by the Jewelers of America in the 1950s:
January - Garnet
February - Amethyst
March - Aquamarine
April - Diamond
May - Emerald
June - Pearl or Moonstone
July - Ruby
August - Peridot
September - Sapphire
October - Opal
November - Citrine
December - Turquoise (or Blue Topaz)