Certain gemstones have more vibration power at certain times of the year. The month of a stone's power time determines your birthstone (also referred to as natal stone).

Some believe that the original associations between gemstones and the months of the year evolved from 12 stones mentioned in the Bible, others say it represents the Twelve Apostles. Still others hold different beliefs. There are numerous traditions over the centuries that connect a particular gemstone with a month of the year.
For this reason, you will notice some stones listed under several months (all the stones associated with each month - whether traditional, talismanic, or other). 

Month January
Garnet, Emerald, Rose Quartz, Onyx, Coral

Month February
Amethyst, Bloodstone, Onyx, Jasper, Imperial Topaz

Month March
Aquamarine, Bloodstone, Jade, Hematite, Ruby

Month April
Diamond, Opal, Crystal Quartz, White Sapphire, Moonstone

Month May
Emerald, Sapphire, Agate, Chrysoprase, Beryl, Carnelian, Garnet

Month June

Pearl, Moostone, Opal, Emerald, Turquoise

Month July
Ruby, Carnelian, Turquoise, Onyx

Month August
Peridot, Sardonyx, Diamond, Sapphire, Jade, Emerald, Aventurine

Month September
Sapphire, Agate, Moonstone, Lapis Lazuli, Diamond, Emerald, Peridot, Aquamarine

Month October
Opal, Tourmaline, Jasper, Pink Tourmaline, Zircon, Aquamarine, Agate, Garnet

Month November
Yellow Topaz, Citrine, Diamond, Tiger Eye, Amethyst, Amber, Aquamarine, Sapphire, Peridot

Month December
Blue Topaz, Turquoise, Zircon, Lapis Lazuli, Onyx, Ruby, Bloodstone, Chrysoprase