Known as zereshk in Iran, these tart red berries are the fruit of a shrub, Berberis vulgaris or B. aristata. They are considered too sour to eat raw. Typically they are purchased in their dried state, reconstituted in water and added to stews and rice dishes to impart a tart taste and jewel-like color. Look for red berries at the market; darker ones may be old. To use, stem the berries, then place them in a colander set in a bowl of cold water for about 20 minutes to allow any sand to sink to the bottom. Lift them out of the water, rinse again with cold water and drain.