Amethyst
AMETHYST, a violet or purple transparent variety of quartz (silicon dioxide) used as an ornamental or gem stone (Rev 21:20). The color is attributed usually to the presence of manganese, or possibly is due to organic material. The ancient Egyptians used it as a gem stone and a common use in antiquity was for intaglios. Amethyst occurs either in mineral veins associated with granites or as a lining of hollow agate (q.v.) nodules.