The gemstone is the birthstone for October. Amethyst is considered the birthstone of February. Peridot is the birthstone for those born in August. Amethyst is the birthstone for those born in February. The Shelby Gem Factory also manufactures simulated citrine and topaz, along with other birthstone substitutes. A birthstone is a gemstone that represents a person's month of birth. The American Gem Trade Association added tanzanite as a December birthstone in 2002. Aquamarine is the birthstone for those born in March. Garnet is the birthstone for those born in January. Sapphire is the birthstone for September and the gem of the 45th anniversary. Sapphire is the birthstone for those born in September. Opal is considered the birthstone for people born in October. The ruby is the birthstone for the month of July. She has said that she chose Diamond because it's her birthstone. The pearl is the birthstone for the month of June.