There are hundreds of plum varieties cultivated throughout the world. All grow in clusters, have smooth, deeply colored skin and a center pit. Plums can range in shape from oval to round and in size from 1 to 3 inches in diameter. Their color can be yellow, green, red, purple, indigo blue and almost anything in between. The pale silvery-gray, filmy-looking coating on a plum's skin is natural and doesn't affect quality. Fresh plums are available from May to late October. Choose firm plums that give slightly to palm pressure. Avoid those with skin blemishes such as cracks, soft spots or brown discolorations, the latter indicating sunburn. Very firm plums may be stored at room temperature until slightly soft. Plums contain a fair amount of vitamin A and potassium.