crown jewel
noun[C]
ukus(alsojewel in the crown)the mostimportantorvaluablepartof something,especiallytheproductorpartof acompany, etc. that makes the mostmoney:
Dairyfarming,longthecrownjewelof Minnesotaagriculture, isleavingthestateat analarmingrate.
Thehigh-speedtrainwill be thejewelin thecrownof thetransportsystem.