(gemology) A precious gem, an iridescent gemstone found in the opal-silica mineral substrate (potch)
2012 March, Lee A. Groat, “Gemstones”, in American Scientist, volume 100, number 2, archived from the original on 14 June 2012, page 128:
Although there are dozens of different types of gems, among the best known and most important are […] . (Common gem materials not addressed in this article include amber, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.)