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, <WSD>turquoise</WSD> and zircon.) 
Meaning of the word marked with <WSD> tags:  A sky-blue, greenish-blue, or greenish-gray semi-precious gemstone.