Sentence:  Although there are dozens of different types of gems, among the best known and most important are diamond, ruby and <WSD>sapphire</WSD>, emerald and other gem forms of the mineral beryl, chrysoberyl, tanzanite, tsavorite, topaz and jade. 
Meaning of the word marked with <WSD> tags:  A clear deep blue variety of corundum, valued as a precious stone.