In 1912, the Jewelers of America national association adopted an official list of 12 birthstones, one to represent each month of the year. This list is internationally recognized and pairs months and birthstones as follows: January–garnet; February–amethyst; March–aquamarine; April–diamond; May–emerald; June–pearl; July–ruby; August–peridot; September–sapphire; October–opal; November–topaz; December–turquoise. As to how the birthstones were originally selected, there are several different theories.
One account traces the concept of the 12 birthstones to the Book of Exodus in the Old Testament, wherein a high priest named Aaron created a breastplate with 12 stones to represent the 12 tribes and sons of Israel. The stones were said to be set in gold filigree and arranged in four rows of three each, as follows: sardius (carnelian), topaz and carbuncle; emerald, sapphire and diamond; ligure (jacinth), agate and amethyst; beryl, onyx and jasper.
Another school of thought links the 12 birthstones to the 12 apostles of Christianity. Historians indicate that between the eighth and ninth centuries, a specific stone was assigned to each apostle, although more than one stone is mentioned in some cases: Peter–jasper; Andrew–ruby/carbuncle; James and John–emerald; Philip–carnelian (sardius); Bartholomew–peridot/chrysolite; Thomas–aquamarine/beryl; Matthew–amethyst/topaz; James–sardonyx; Thaddeus–chrysoprase; Simon–zircon/jacinth; Matthias–amethyst; and Paul–sapphire.
It is also possible that the concept of birthstones comes from the Bible’s Book of Revelation, and were inspired by a list of foundation stones that symbolized the new Jerusalem. They are listed in an order according to the Roman calendar: jasper, sapphire, chalcedony, emerald; sardonyx, sardius (carnelian), chrysolite, beryl, topaz, chrysoprase, jacinth and amethyst.
The birthstones now linked to each of the 12 months of the year may have been selected according to astrological beliefs. According to ancient Tibetan and Indian cultures, certain stones were thought to have mystical and magical powers assigned to them. Therefore, they were worn on the person to draw upon their perceived powers.
It is believed that the custom of wearing birthstones may date back to around the 1600s or 1700s in Poland. Today, one birthstone is assigned to each month and to one astrological sign. It was once the thinking that people should wear every one of the 12 stones during the appointed months so as to derive the full benefits of every stone throughout the calendar year.