In the Bible, in the book of Exodus, a High Priest's breastplate is described. Exodus 28: 15-30 reads:
And thou shalt make the breastplate of judgment with cunning work; after the work of the ephod thou shalt make it; of gold, of blue, and of purple, and of scarlet, and of fine twined linen shalt thou make it. Foursquare it shall be being doubled; a span shall be the length thereof, and a span shall be the breadth thereof. And thou shalt set in it settings of stones, even four rows of stones: the first row shall be a sardius, a topaz, and a carbuncle: this shall be the first row. And the second row shall be an emerald, a sapphire, and a diamond. And the third row a ligure, an agate, and an amethyst. And the fourth row a beryl, and an onyx, and a jasper; they shall be set in gold in their enclosings. And the stones shall be with the names of the children of Israel, twelve, according to their names, like the engravings of a signet; every one with his name shall they be according to the twelve tribes. And thou shalt make upon the breastplate chains at the ends of wreathen work of pure gold. And thou shalt make upon the breastplate two rings of gold, and shalt put the two rings on the two ends of the breastplate. And thou shalt put the two wreathen chains of gold in the two rings which are on the ends of the breastplate. And the other two ends of the two wreathen chains thou shalt fasten in the two ouches, and put them on the shoulder-pieces of the ephod before it. And thou shalt make two rings of gold, and thou shalt put them upon the two ends of the breastplate in the border thereof, which is in the side of the ephod inward.
And two other rings of gold thou shalt make, and shalt put them on the two sides of the ephod underneath, toward the forepart thereof, over against the other coupling thereof, above the curious girdle of the ephod. And they shall bind the breastplate by the rings thereof unto the rings of the ephod with a lace of blue, that it may be above the curious girdle of the ephod, and that the breastplate be not loosed from the ephod. And Aaron shall bear the names of the children of Israel in the breastplate of judgment upon his heart, when he goeth in unto the holy place, for a memorial before the Lord continually. And thou shalt put in the breastplate of Judgment the Urim and the Thummim; and they shall be upon Aaron’s heart, when he goeth in before the Lord: and Aaron shall bear the judgment of the children of Israel upon his heart before the Lord continually.
(My own note: it's important to keep in mind that the above is a translation of an incredibly old text in a language that was mostly dormant since about 400 CE. The revival of Hebrew is relatively recent and there are still many words that we don't have perfect translations for. We don't know for certain what the exact stones are on the breastplate. These are educated guesses and there are many different speculations. For example, the ancient Israelites were not rich people, so getting some of the more expensive gems seems unlikely.)
This breastplate was used as a sort of divination tool. It's said that it would emit light when YHWH was present at sacrifices and when YHWH was present to aid the people in battle. Regarding the adamas (the diamond, from what I understand), the following is stated: If the people were sinful and disobedient, the stone assumed a dusky hue, which portended death by disease, or else it became the color of blood, signifying that the people would be slain by the sword. If, however, the stone shone like the driven snow, then the people recognized that they had not sinned, and hastened to celebrate the festival. When an accused criminal's name was said, the stones would "lose their light and grow dim," thus proving the person's guilt.
Alright, that‘s it for this section of the book. I should have the next part out next Sunday, so make sure to check back if you‘re interested!