It should be a simple answer, but I haven't been able to get a
conclusive result.  Can someone please tell me exactly what kind of
stones are "Urim and Thummim"?  I am already aware of their use and
such information isn't necessary.  All I need is what kind of stones
they are or, if they're not naturally occuring.  But it needs to be a
conclusive answer.

Thanks,

I do not believe it is possible to give a "conclusive answer" to this
question. Speculation, yes. But certainty just isn't an option.

Urim and Thummim. From information in the Bible, these were two '
Objects' used by the Israelites, led by Moses, in their worship. The
Bible does not state what 'type' of stone or other material they were
made of, but does indicate their purpose. They could have been used as
' lots' for decisions. [ Exodus 28;30] They may have been two flat
stones, one white and one black, tossed like dice. Two white sides up
meant Yes, two black, No, black and white, no answer. It is possible
that God could give an answer to a problem to Moses by the use of such
a device. First Samuel 14;36-42 gives an indication. "O God of Israel,
do give Thummim". From God's point of view, what they were made of is
not important, but the purpose is.