From Ginsberg.....
The twelve stones in the breastplate, with their bright colors, were of great importance in the oracular sentences of the high priest, who by means of these stones made the Urim and Tummim exercise their functions. For whenever the king or the head of the Sanhedrin wished to get directions from the Urim and Tummim he betook himself to the high priest. The latter, robed in his breastplate and ephod, bade him look into his face and submit his inquiry. The high priest, looking down on his breastplate, then looked to see which of the letters engraved on the stones shone out most brightly, and then constructed the answer out of these letters. Thus, for example, when David inquired of the Urim and Tummim if Saul would pursue him, the high priest Abiathar beheld gleaming forth the letter Yod in Judah's name, Resh in Reuben's name, and Dalet in Dan's name, hence the answer read as follows: Yered, "He will pursue."
The information of this oracle was always trustworthy, for the meaning of the name Urim and Tummim is in the fact that "these answers spread light and truth," but not every high priest succeeded in obtaining them. Only a high priest who was permeated with the Holy Spirit, and over whom rested the Shekinah, might obtain an answer, for in other cases the stones withheld their power. But if the high priest was worthy, he received an answer to every inquiry, for on these stones were engraved all the letters of the alphabet, so that all conceivable words could be constructed from them. [358]
http://philologos.org/__eb-lotj/vol3/p0 ... REASTPLATE
