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, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and <WSD>zircon</WSD>.) 
Meaning of the word marked with <WSD> tags:  A mineral occurring in tetragonal crystals, usually of a brown or grey colour and consisting of silica and zirconia.