Birthstone

Gemstone associated with the month of one's birth. Wearing one was popularly supposed to bring good luck or health. The ancient belief that supernatural powers could be attributed to gemstones is still held by some people today.

From http://www.britannica.com/

Monthly Birthstone:

January garnet
February amethyst
March aquamarine
April diamond
May emerald
June moonstone, pearl, alexandrite
July ruby
August Peridot
September sapphire
October opal
November topaz, citrine
December turquoise, zircon