Commonly, a dried plum. Only certain varieties of plums can be covered to prunes. In the fresh, fully ripe state, the plums used to make prunes are dark blue to purple and oblong or oval, They have firm flesh and are high in sugar. When dried, pit and all, the meat does not ferment or spoil as do other types of plums. Prunes have a very sweet, rich flavor that is more intense than that of fresh plums.