Catalyzes the cyclization of farnesyl diphosphate to multiple sesquiterpenes, such as olefins and sesquiterpene alcohols. Binds 3 Mg(2+) ions per subunit. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.