ANSWER: The high priest's breast plate (or piece) has twelve stones on it. They represent each of Israel's twelve tribes. In this case, the Bible tells us clearly the symbolism involved and their setting within the breastplate:
15 Make a breastpiece for the High Priest to use in determining God’s will. It is to be made of the same materials as the ephod and with similar embroidery. 21 Each of these twelve stones is to have engraved on it the name of one of the sons of Jacob, to represent the tribes of Israel. (Exodus 28)
The stones, which had to be precious, were to be mounted on four rows. Row number one had a garnet, topaz and ruby. Row number two contained a sapphire, diamond and emerald. Row number three had an agate, amethyst and turquoise stone. Row four, the final row, had placed in it a jasper, beryl and carnelian stone. Additionally, God commanded that all the stones be mounted in GOLD.
It is not obvious, however, which stone represents which tribe. I'm not aware of them representing a single letter in the Hebrew alphabet. The Hebrew alphabet has several more letters than twelve, it should be noted.
The order of the jewels is not fully clear, although one would assume the order listed for each of the four rows would run from right to left, as Hebrew does, rather than left to right, as English does. The order is repeated again in Exodus 39:8 - 14.
It's not obvious what the significance, if any, is of the order of the precious jewels. There could be more symbolism here than the obvious points raised above. Duality in Biblical symbols and prophecy is common.