Stones associated with the month of birth. The practice may date back to the Breastplate of Aaron that had twelve stones. There have been many sets of such stones, but in 1912 the association known as the Jewelers of America (some cynics say to increase their income) created this list:

January.............Garnet
February............Amethyst
March..................Aquamarine
April.....................Diamond
May....................Emerald
June..................Pearl or Moonstone
July...................Ruby
August..............Peridot
September........Sapphire
October............Opal
November ........Yellow Topaz
December.........Turquoise or Blue Topaz