The URIM and THUMMIN were 2 gemstones they put in a pouch. One was red and the other was green. They put their hand in the pouch and pull one out and the one that they pulled out was considered to be God’s will.
There are many oral and written traditions on this, but as mentioned already… it isn’t known for sure what the properties were of these items. It’s really speculation that they were red or green or black or white… the only known physical properties were that they were kept in a breastplate…so… “smaller than a bread basket” to quote that favourite child-hood game.