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, <WSD>spinel</WSD>, tourmaline, turquoise and zircon.) 
Meaning of the word marked with <WSD> tags:  Any of several hard minerals of cubic symmetry that are mixed oxides of magnesium and aluminium and are used as gemstones of various colours.