There is a considerable amount of discussion to be made concerning the Urim and Thummim that are mentioned in the Holy Bible. The etymology of these terms is unclear. But regardless, of how they originated, what material they were made from or even of their definite meaning is not necessary to know in order to understand how they were used.
Following are scriptures where the terms are used.
Exodus 28:30; “You shall put in the breastpiece of judgment the Urim and the Thummim, and they shall be over Aaron’s heart when he goes in before the LORD; and Aaron shall carry the judgment of the sons of Israel over his heart before the LORD continually.”
Leviticus 8:8; And he put the breastplate upon him: also he put in the breastplate the Urim and the Thummim.
Deuteronomy 33:8; “And of Levi he said, Let thy Thummim and thy Urim be with thy holy one, whom thou didst prove at Massah, and with whom thou didst strive at the waters of Meribah.”
Numbers 27:21; “Moreover, he shall stand before Eleazar the priest, who shall inquire for him by the judgment of the Urim before the LORD. At his command they shall go out and at his command they shall come in, both he and the sons of Israel with him, even all the congregation.” (Notice the Thummim is not mentioned in this passage)
1 Samuel 23:7-13; When it was told Saul that David had come to Keilah, Saul said, “God has delivered him into my hand, for he shut himself in by entering a city with double gates and bars.” So Saul summoned all the people for war, to go down to Keilah to besiege David and his men. Now David knew that Saul was plotting evil against him; so he said to Abiathar the priest, “Bring the ephod here.” Then David said, “O LORD God of Israel, Your servant has heard for certain that Saul is seeking to come to Keilah to destroy the city on my account. “Will the men of Keilah surrender me into his hand? Will Saul come down just as Your servant has heard? O LORD God of Israel, I pray, tell Your servant.” And the LORD said, “He will come down.” Then David said, “Will the men of Keilah surrender me and my men into the hand of Saul?” And the LORD said, “They will surrender you.” Then David and his men, about six hundred, arose and departed from Keilah, and they went wherever they could go. When it was told Saul that David had escaped from Keilah, he gave up the pursuit. (These scriptures, although they do not mention the Urim and Thummim, may have been attached to the ephod that David called for.)
And the Tirshatha said unto them, that they should not eat of the most holy things, till there stood [up] a priest with Urim and Thummim. (Ezra 2:63) (Nehemiah 7:65)
The above references (from the KJV) contain all that is said in the Holy Scriptures concerning these two substances. There are certain things which become obvious by these passages while other things remain a mystery. If we may presume that Urim and Thummim were attached to David’s ephod, then it becomes obvious that they offered access to seeking some facts from God Himself.
First and foremost, the purpose for the two substances reflect that they were ordered by God to be used as a means of obtaining truths or facts when those things were needed, yet unknown. And even though the text implies that God literally spoke the answers to David, it does not absolutely require it. It may have been that placed the answer into David’s psychic.
We find this information concerning the origin of the two terms:
תוּמִים (Thummim) is widely considered to be derived from the consonantal root תתּוּמִים(t-m-m), meaning innocent, while אוּרִים (Urim) has traditionally been taken to derive from a root meaning lights; these derivations are reflected in the Neqqudot of the masoretic text. In consequence, Urim and Thummim has traditionally been translated as lights and perfections (by Theodotion, for example), or, by taking the phrase allegorically, as meaning revelation and truth, or doctrine and truth (it appears in this form in the Vulgate, in the writing of Jerome, and in the Hexapla). It should be understood that “Thummim” is pronounced /tumim/ in Modern Hebrew (Urim and Thummim: Wikipedia online Encyclopedia)
So, even though the words have changed and the meanings may have changed somewhat, they apparently began with the meanings of “Innocent or Innocence” for Urim and “lights and perfections” for Thummim. These meanings, according to a number of scholars, may readily have changed to more recognizable terms. For instance, many think the meaning of Urim also reflected as “pure, factual, or without blemish or error”. Thummim, with certain similarities but bringing more understanding of the term may have had underlying meanings of “complete truth, understanding, etc“.
It seems quite logical, to this writer, that these could have well been a means of receiving truthful answers from God over questions such as David asked. And in the case of the high priest, (Aaron, Eleazar, etc), they were worn over their hearts in the breastplate which was attached to the upper portion of the ephod. This was to be worn as they dedicated and consecrated the various sacrifices and also when they entered the Most Holy place of the Tabernacle, where they were in the presence of God. Their purpose seems to have been to assure they performed everything exactly to the letter of God’s instruction. It is not known whether or not the high priest had to ask for any instructions which he might have found unclear or if God simply directed his actions via those items, much as the Holy Spirit guided the inspired Apostles after He was sent to them by a risen Jesus.
The most totally unanswered question concerning the Urim and Thummim remains their physical properties. There seems to be no record of what was used for these items. Some have conjectured that they were 12 small stones with the names of the 12 tribes on them. Others feel they were simply small precious gems and others even that they may have been tiny chips of some certain types of wood. But certainly up to this point, all of these are nothing more than conjecture.
Scholars and Historians (Including Flavius Josephus) agree that there is no indication of these items ever being mentioned or used after the destruction of the first temple. After the land of Canaan was successfully taken by the 12 tribes and the first temple was constructed by Solomon, these emblems were kept in the temple. Seemingly, there may have been only one set of them.