The jewel in the crown is the most precious part of some possession. The expression was used frequently to describe India during the time of the British Raj. Prime Minister Disraeli had arranged for Queen Victoria to be named Empress of India and so added this costly gem to her crown, which represented the ,then, vast British Empire. The cherry on the cake is something which children often squabble over, and has come to mean a special treat or some particularly desirable perquisite (perk) that comes with a job or other undertaking. For instance, if you got a good job in which you were allocated a Mercedes as a company car, or if your duties involved spending three months in Hawaii every year, that would be the cherry on the cake.