Among the leading plum varieties are the European plum, which is medium-sized and dark blue to red, with a thick skin and dull yellow flesh. This variety is often dried into prunes. The Japanese plum can be purple, scarlet, or yellowish-green and is sweet and juicy. Damson and Mirabelle plums make delicious jam but are quite tart. Over 35 percent of the California plum harvest is of the Santa Rosa variety. Casselman, Elephant Heart, El Dorado, Greengage, and Laroda are other popular varieties.