“Also, that there were two stones in silver bows—and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim—deposited with the plates; and the possession and use of these stones were what constituted Seers in ancient or former times; and that God had prepared them for the purpose of translating the book." Book of Mormon, Introduction

When Joseph Smith was visited in his room on September 21, 1823, the angel Moroni told him about two stones, Urim and Thummim, that would help him translate the gold plates.  He was told not to show them to anyone, unless Moroni told him to show it to them.

These stones apparently had the ability to see things, translate languages and communicate directly from God.

These stones appear in the Bible and they have a very different purpose.

Also put the Urim and the Thummim in the breastpiece, so they may be over Aaron’s heart whenever he enters the presence of the LORD. Thus Aaron will always bear the means of making decisions for the Israelites over his heart before the LORD.  Exodus 28:30

The Urim and Thummim were stones that were secured on the breastplate of the High Priests.  They were used to make decisions, kind of like casting lots.  Since people were not filled with the Holy Spirit, a guide and teacher, people had to use other methods for guidance.  They believed God would make the decision through chance.

This would be likened to drawing straws or rolling dice or rock-paper-scissors.

The Urim and Thummin had no other special traits as Moroni and Joseph Smith claimed they had.