Dora Bas Rivka Silver O'H
Urim and Tumim
The two gems of the ephod were to be placed in settings of gold with gold epaulets.
The breastplate was to have a pattern woven like that of the ephod. There would be four rows of three gems each, engraved with the names of Yaakov's sons, one per stone. (According to the Talmud in Yoma 73b, the names of the Patriarchs and the words "shivtei Yeshurun" - "the upright Tribes" - were also on the stones, giving them the entirety of the Hebrew alphabet.) The breastplate was to be attached to the ephod by gold cords. This way, Aharon would also be wearing the names of the Tribes over his heart when he served before G-d.
The breastplate were to contain something called Urim and Tumim, which were used to consult G-d on important matters. The Urim and Tumim are never defined, but traditionally they were a parchment containing the Names of G-d, which would cause the letters on the stones to light up with messages. (The name "Urim and Tumim" comes from the Hebrew words for light and perfection.)