Sentence:  Although there are dozens of different types of gems, among the best known and most important are diamond, ruby and sapphire, <WSD>emerald</WSD> and other gem forms of the mineral beryl, chrysoberyl, tanzanite, tsavorite, topaz and jade. 
Meaning of the word marked with <WSD> tags:  Any of various green gemstones, especially a green transparent form of beryl, highly valued as a precious stone.