Kidney stones. X-ray of the kidney of a patient with kidney stones (dark areas) in the renal pelvis (upper centre) of the kidney. Kidney stones, or calculi, are formed when mineral salts, most often involving calcium, accumulate in the inner surfaces of the kidney. As they grow, they may block the flow of urine out of the kidney, causing extreme pain. Most stones are small enough to be passed, but larger stones need to be treated. They may be removed by surgery or using non-invasive means such as applying sound waves to break up the stone.