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