According to the Gemological Institute of America (ref. G.I.A. Colored Stones Course, Assignment #41, "Birthstones"), the origin of the Western system of birthstones can be traced to the Breastplate of the High Priest. In the Bible, Moses handed down rules governing the Breastplate of Judgment, which contained 12 gems, each engraved with the name of one of the twelve tribes of Israel. Another arrangement of the 12 gems is described in the Book of Revelation as the foundation stones of the New Jerusalem. The order in which these 12 gems were given was used to determine the succession of birthstones for the months.

Of Interest. . .

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.

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.