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, <WSD>opals</WSD>, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.) 
Meaning of the word marked with <WSD> tags:  A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO<sub>2</sub>'''·'''nH<sub>2</sub>O.