The Hebrew word achlamah refers to the amethyst, a violet or purple variety of quartz prized as a precious gemstone. The word appears in the description of the high priest's breastplate (choshen), where it was one of the twelve stones set in gold filigree — each stone engraved with the name of one of the twelve tribes of Israel.
The high priest's breastplate (choshen) was a masterpiece of sacred craftsmanship bearing twelve precious stones engraved with the names of the twelve tribes of Israel. The priest wore this over his heart as he entered God's presence, symbolizing that he carried all of God's people before God in intercession. The amethyst, with its royal purple hue, speaks of the dignity and worth God places upon each tribe. In Revelation 21:20, amethyst appears again as one of the foundation stones of the New Jerusalem, linking the old covenant breastplate to the new covenant city that descends from heaven.