dried lime
Whole limes sun-dried until hard, hollow and brown to black; they give stews a sharp, almost smoky sourness.
Dried limes, known in Persian as limoo omani, are small limes boiled whole in brine and then dried in the sun until skin and flesh turn hard and papery-light. They taste sour, but unlike fresh limes: deep, slightly fermented, with a musty, somewhat bitter edge reminiscent of tea or smoke. Before cooking they are pierced a few times or cracked open and left to simmer for a long time in stews and soups, or they are ground to a powder and sprinkled over rice and meat. Pale ones are milder, dark to black ones stronger and more bitter. They are sold in Persian, Arab and Turkish grocery stores, often also ground, and through spice mail-order.
Substitute:
There is no truly good substitute. Fresh lime juice with a little grated zest, added towards the end, supplies the sourness but not the fermented, slightly bitter depth that defines the dish; a little sumac comes somewhat closer to the tart character but is milder.