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.)
2012 March, Lee A. Groat, “Gemstones”, in American Scientist, volume 100, number 2, archived from the original on 14 June 2012, page 128