In chapter 28 of Exodus we read about the ephod of Aaron which details six garments that make up the "Priests"  garments to be worn during the performance of his office.

In the narrative we learn of 14 different stones; each with a specific placement on the "Breastplate of Judgment"

21 then 29 versus 30. 

Note that in these scriptures there is no specific amount given for the Urim and Thummim.  There is no color give.  There is no mention of engraving.  There is no separate instruction for their purpose, makeup, or specific placement on the breastplate.

Note that verse 29 and verse 30 are worded almost exactly the same.

Read verse 15 again and then compare the first clause with that of verses 29 and 30.

Could the Urim and Thimmim be the 12 stones, in four rows of three, that are described in detail upon the "breastplate of righteousness"?

I believe they are and therefore are not additions to the breastplate - nor are they separate.

If they are not the same 12 stones, then why are they not described in at least some detail.

Everything else about the tabernacle, ark, Holy of Holies, Ephod, furniture, and many other things are described in great detail. 

Is it reasonable to think that these two things should be introduced without any details?

I think not.  I think they are, then, those twelve stones that represent the 12 tribes; the stones described in detail and worn on the "breastplate of judgment".

Are they not used to make judgments throughout the scriptures?  But, there is never any details about them.

Your thoughts....