Sentence:  Although there are dozens of different types of gems, among the best known and most important are .... (Common gem materials not addressed in this article include <WSD>amber</WSD>, amethyst, chalcedony, garnet, lazurite, malachite, opals, peridot, rhodonite, spinel, tourmaline, turquoise and zircon.) 
Meaning of the word marked with <WSD> tags:  A hard, generally yellow to brown translucent fossil resin, used for jewellery. One variety, blue amber, appears blue rather than yellow under direct sunlight. (from 15th c.)