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.