John Parkhurst, in his Hebrew lexicon titled "A Hebrew and English Lexicon Without Points", argues that the Urim and Thummim were the same as the 12 stones on the breastplate. I have quoted most of his lexicon entry below. Note that I have made some minor changes for easier reading, such as changing his abbreviated Scripture references to full references, and Roman numeral chapter numbers to Arabic numerals (e.g. "Exod. xxviii" to "Exodus 28"). I have also inserted Scripture quotations directly within his lexicon entry for quick reference to most of his mentioned passages. Text within brackets [ ] are also mine.
IV. As a noun masculine plural אורים.
את האורים ואת התמים URIM and THUMMIM, lights and perfections, mentioned Exodus 28:30. Leviticus 8:8,
Exodus 28:30 (NKJV)
And you shall put in the breastplate of judgment the Urim and the Thummim, and they shall be over Aaron’s heart when he goes in before the Lord. So Aaron shall bear the judgment of the children of Israel over his heart before the Lord continually.
Leviticus 8:8 (NKJV)
Then he put the breastplate on him, and he put the Urim and the Thummim in the breastplate.
as some things that were put into the breast-plate of the high-priest. That these did in some manner or other give prophetical or oracular answers from Jehovah is disputed by none, who pretend to believe the authority of the Scriptures, being evidently proved from Numbers 27:21. 1 Samuel 28:6, & al.
Numbers 27:21 (NKJV)
He [Joshua the son of Nun] shall stand before Eleazar the priest, who shall inquire before the Lord for him by the judgment of the Urim. At his word they shall go out, and at his word they shall come in, he and all the children of Israel with him—all the congregation.
1 Samuel 28:6 (NKJV)
And when Saul inquired of the Lord, the Lord did not answer him, either by dreams or by Urim or by the prophets.
But the two great questions relating to them are,
1st. Of what form and substance were these Urim and Thummim?
2ndly. How or in what manner prophetic answers were delivered by them?
--Not to trouble the reader with rabbinical dreams [Parkhurst might be referring to things such as Yoma 73b in the Talmud, where Resh Lakish says the letters engraved on the 12 stones physically moved: "...the letters composing the words came near each other."], or what seem to me erroneous opinions on this subject, I shall endeavour to clear both these points from the Scriptures themselves.
1st. As to their form and substance, it seems highly probable that they were no other than the twelve precious stones inserted into the high-priest's breast-plate, (Exodus 28:17, &c.) on which were engraven the names of the twelve tribes of Israel; for,
1st. It is written, Exodus 28:29 [KJV], Aaron shall bear the names of the children of Israel (namely, those engraven on the stones) in the breast-plate of judgment upon his heart, when he goeth into the holy place, for a memorial before the Lord continually. And to enjoin this the more strongly, the same thing is expressed, ver. 30, and thou shalt put in the breast-plate of judgment the Urim and the Thummim, and they shall be upon Aaron's heart when he goeth before the Lord: and (or so) Aaron shall bear the judgment of the children of Israel upon his heart before the Lord continually. Who that compares these two verses attentively together, but must see that the Urim and Thummim are the substance or matter upon which the names were engraven?
2ndly. In the description of the high-priest's breast-plate, given Exodus 39:8, & seq. the Urim and Thummim are not mentioned, but the rows of stones are; and vice versa in the description, Leviticus 8:8, the Urim and Thummim are mentioned by name, and the stones not; therefore it is probable that the Urim and Thummim and the precious stones are only different names for the same thing.
3rdly. If the Urim and Thummim be not the same with precious stones, then we must say that Moses, who hath so particularly described the most minute things relating to the high-priest's dress, hath given us no description at all of this most stupendous part of it, which seems highly improbable.
As to the 2nd question, how, or in what manner prophetic answers were delivered by Urim and Thummim? It seems determined, beyond dispute, that it was by an audible voice, as at other times; (Numbers 7:89.) for when David consulted Jehovah by the ephod of Abiathar, we read 1 Samuel 23:11, Jehovah אמר said, he will come down. So again verse 12.
1 Samuel 23:11-12 (NKJV)
11 Will the men of Keilah deliver me [David] into his [Saul's] hand? Will Saul come down, as Your servant has heard? O Lord God of Israel, I pray, tell Your servant.”
And the Lord said, “He will come down.”
12 Then David said, “Will the men of Keilah deliver me and my men into the hand of Saul?”
And the Lord said, “They will deliver you.”
Compare also 1 Samuel 30:7, 8. 2 Samuel 2:1; 5:23, 24. Judges 1:1, 2; 20:18.
1 Samuel 30:7, 8 (NKJV)
7 Then David said to Abiathar the priest, Ahimelech’s son, “Please bring the ephod here to me.” And Abiathar brought the ephod to David. 8 So David inquired of the Lord, saying, “Shall I pursue this troop? Shall I overtake them?”
And He answered him, “Pursue, for you shall surely overtake them and without fail recover all.”
2 Samuel 2:1 (NKJV)
1 It happened after this that David inquired of the Lord, saying, “Shall I go up to any of the cities of Judah?”
And the Lord said to him, “Go up.”
David said, “Where shall I go up?”
And He said, “To Hebron.”
2 Samuel 5:23, 24 (NKJV)
23 Therefore David inquired of the Lord, and He said, “You shall not go up; circle around behind them, and come upon them in front of the mulberry trees. 24 And it shall be, when you hear the sound of marching in the tops of the mulberry trees, then you shall advance quickly. For then the Lord will go out before you to strike the camp of the Philistines.”
Judges 1:1, 2 (NKJV)
1 Now after the death of Joshua it came to pass that the children of Israel asked the Lord, saying, “Who shall be first to go up for us against the Canaanites to fight against them?”
2 And the Lord said, “Judah shall go up. Indeed I have delivered the land into his hand.”
Judges 20:18 (NKJV)
Then the children of Israel arose and went up to the house of God [Or Bethel] to inquire of God. They said, “Which of us shall go up first to battle against the children of Benjamin?”
The Lord said, “Judah first!”
Thus then it was Jehovah who returned an answer by an audible voice, when the priest presented himself before him with the Urim and Thummim.
Who can doubt but the typical high-priest's appearing continually before Jehovah with the names of the children of Israel upon his heart prefigured the appearing of the real High-priest in the presence of God, as intercessor for ever, in behalf of the true Israel, even of all those who come unto God by him? Who can doubt but that Jehovah's being sometimes (see 1 Samuel 28:6) pleased to answer by Urim and Thummim, was a shadow of that spirit of truth and prophecy which was to be inherent in Jehovah incarnate? See Deuteronomy 33:8.
Deuteronomy 33:8 (NKJV)
And of Levi he said:
“Let Your Thummim and Your Urim be with Your holy one,
Whom You tested at Massah,
And with whom You contended at the waters of Meribah,
There was a remarkable imitation of this sacred ornament among the Egyptians, for we learn from Diodorus, lib. i. p. 68, Rhod. and from Ælian, Var. Hist. lib. xix. cap. 34, that "their chief-priest, who was also their supreme judge in civil matters, wore about his neck, by a golden chain, an ornament of precious stones called Truth (Αληθεια, the very word by which the LXX render תמים [Thummim] Exodus 28:33. Leviticus 8:8.) and that a cause was not opened till the supreme judge had put on this ornament."
I think Parkhurst has adequately proved that the manner in which prophetic answers were given was indeed by an audible voice, and as such rules out any kind of sacred lot.
Josephus mentions that the 12 stones on the breastplate shined brightly before the Israelite army began to march, which assured the people that they would be victorious:
Note how the Egyptian amulet was made up of multiple precious stones, yet it's name was called Truth. In a similar manner the 12 stones in the breastplate of the high-priest might have been called Urim and Thummim.
Unfortunately, while Parkhurst does make note of the aforementioned passages, he does not address this particular issue to my satisfaction.
While there is some evidence that "Urim and Thummim" was the name for the precious stones in the breastplate of the high-priest, there is also evidence against it. So unfortunately we cannot know for certain that "Urim and Thummim" was just the name/title for the 12 stones that were mounted in the breastplate.