Most gem scholars agree that the tradition of birthstones arose from the Breastplate of Aaron: a ceremonial religious garment set with twelve gemstones that represented the twelve tribes of Israel and also corresponded with the twelve signs of the zodiac and the twelve months of the year.
Because ancient people did not always classify gemstones by mineral species like we do, there is some debate about which gemstones were set in the breastplate and why. Because of this, different cultures around the world have developed different birth stone lists. The modern day list that you know is only the most recent list: some older lists still exist. Some also argue that the proper way to assign gemstones is according to astrological sign and not month. We think it is more fun to choose the gemstone that speaks to you from all the possibilities. Of course it is hard to keep track of all the lists. Enter the Gem-o-Matic! Select your birthdate or other significant date or anniversary and the Gem-o-Matic will give you the list of all the birthstones that correspond to that date!
The instructions for fabricating the Breastplate of the High Priest, or the Breastplate of Aaron, can be found in Exodus 28, 15-30:
And thou shall make the breastplate of judgement with cunning work; after the work of the ephod thou shalt make it; of gold, of blue, and of purple, and of scarlet, and of fine linen shalt thou make it.ICA
Foursquare it shall be doubled; a span shall be the length thereof, and a span shall be the breadth thereof.
And thou shalt set in it settings of stones, even four rows of stones: the first row shall be a sardius, a topaz, and a carbuncle: this shall be the first row.
And the second row shall be an emerald, a sapphire, and a diamond. And the third row a ligure, an agate, and an amethyst. And the fourth row a beryl, and an onyx, and a jasper; they shall be set in gold in their enclosings.
And the stones shall be with the names of the children of Israel, twelve, according to their names, like the engravings on a signet; every one with his name shall they be according to the twelve tribes....
And Aaron shall bear the names of the children of Israel in the breastplate of judgement upon his heart, when he goeth in unto the holy place, for a memorial before the Lord continually.