Mohammedan legend represent the heavens with different stones and gems. In the middle ages religious ideas become intertwined with astrological and medical practices.
Stones do appear in the Bible text throughout.
Amethyst is one of the stones mentioned. In Exodus xxviii, 15-30 the Breastplate of judgement had an Amethyst in the third row. God announced victory in battle by means of the twelve stones worn by the high priest on his breast. Andreas, Bishop of Caesarea wrote about St John in Revelations xxi, 19. He wrote that Amethyst signifies Matthias, who in the gift of touges was so filled with celestial fire and with ferent zeal to serve and please God, who had chosen him, that he was found worthy to take the place of the apostate Judas.