What we know as amethyst (not necessarily the same stone).
A precious stone in the third row of the breastplate of the high priest (Ex. 28:19; Ex. 39:12); and the twelfth stone in the foundations of the wall of the heavenly Jerusalem (Rev. 21:20). This gem is not definitely identified. It may be the purple variety of corundum, known as the oriental amethyst, or the transparent amethystine quartz which contains oxide of manganese and alumina. The Hebrew word is achlamah which is translated ἀμέθυστος in the above two passages in Exodus by the LXX, who also have the same in Ezekiel 28:13, where there is nothing in the Hebrew.