Sardonyx

Sardonyx, a variety of onyx (q.v.), or banded chalcedony, consisting of white or blue-grey layers alternating with red (carnelian) or brown (sard), or of all three superposed. When well and sharply coloured it is termed Oriental sardonyx; but the colours are often heightened artificially, or the sardonyx is built up of cemented layers of various chalcedonies. It has always been a favourite material for cameo-engraving, the finest, such as the Triumph of Bacchus and Ceres in the Vatican Museum, which measures 16 inches by 12 inches, being cut in stone of five different layers.