Sentence:  Although there are dozens of different types of gems, among the best known and most important are [...]&thinsp;. (Common gem materials not addressed in this article include amber, <WSD>amethyst</WSD>, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.) 
Meaning of the word marked with <WSD> tags:  A transparent purple variety of quartz, used as a gemstone.