Kidney stone. Coloured frontal X-ray of a male patient with a kidney stone (small white area slightly to the right at lower centre) in his bladder (large dark area at lower centre). Part of the spine is seen at upper centre (white), on either side of which are the kidneys (blue ovals). 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.