Yeah, but there's no description. It also doesn't describe how something can be put into a breastplate. Is there a pocket, setting, loop? It doesn't appear in depictions. Is it a pair of dice? Bag of rocks? Maybe the priest talks into something.
Exodus 28:
The Ephod
6 “They are to make the ephod of finely spun linen embroidered with gold, and with blue, purple, and scarlet yarn. 7 It must have two shoulder pieces attached to its two edges so that it can be joined together. 8 The artistically woven waistband that is on the ephod must be of one piece, according to the same workmanship of gold, of blue, purple, and scarlet yarn, and of finely spun linen.
9 “Take two onyx stones and engrave on them the names of Israel’s sons: 10 six of their names on the first stone and the remaining six names on the second stone, in the order of their birth. 11 Engrave the two stones with the names of Israel’s sons as a gem cutter engraves a seal. Mount them, surrounded with gold filigree settings. 12 Fasten both stones on the shoulder pieces of the ephod as memorial stones for the Israelites. Aaron will carry their names on his two shoulders before the Lord as a reminder. 13 Fashion gold filigree settings 14 and two chains of pure gold; you will make them of braided cord work, and attach the cord chains to the settings.
The Breastpiece
15 “You are to make an embroidered breastpiece for making decisions. Make it with the same workmanship as the ephod; make it of gold, of blue, purple, and scarlet yarn, and of finely spun linen. 16 It must be square and folded double, nine inches long and nine inches wide. 17 Place a setting of gemstones on it, four rows of stones:
The first row should be
a row of carnelian, topaz, and emerald;
18 the second row,
a turquoise, a sapphire, and a diamond;
19 the third row,
a jacinth, an agate, and an amethyst;
20 and the fourth row,
a beryl, an onyx, and a jasper.
They should be adorned with gold filigree in their settings. 21 The 12 stones are to correspond to the names of Israel’s sons. Each stone must be engraved like a seal, with one of the names of the 12 tribes.
22 “You are to make braided chains of pure gold cord work for the breastpiece. 23 Fashion two gold rings for the breastpiece and attach them to its two corners. 24 Then attach the two gold cords to the two gold rings at the corners of the breastpiece. 25 Attach the other ends of the two cords to the two filigree settings, and in this way attach them to the ephod’s shoulder pieces in the front. 26 Make two other gold rings and put them at the two other corners of the breastpiece on the edge that is next to the inner border of the ephod. 27 Make two more gold rings and attach them to the bottom of the ephod’s two shoulder pieces on its front, close to its seam, and above the ephod’s woven waistband. 28 The craftsmen are to tie the breastpiece from its rings to the rings of the ephod with a cord of blue yarn, so that the breastpiece is above the ephod’s waistband and does not come loose from the ephod.
29 “Whenever he enters the sanctuary, Aaron is to carry the names of Israel’s sons over his heart on the breastpiece for decisions, as a continual reminder before the Lord. 30 Place the Urim and Thummim in the breastpiece for decisions, so that they will also be over Aaron’s heart whenever he comes before the Lord. Aaron will continually carry the means of decisions for the Israelites over his heart before the Lord.
Well, it sounds to me like the breastplate is very much like a shaman medicine bag they wear around their necks close to their heart to carry their fetishes, which are often decorated on the outside with beads or stones. (But what do I know? For all I know the breastplate was like a "strike a light bag" and the umin and thummim were like the flint and steele plate carried inside. The usual means of divination would be to designate one as "yes" and one as "no" or one as "this group" and the other as "that group," and drawing one out of the bag. You can then half the chosen group and repeat until you get the one(s) remaining needed. See Cleromancy.)