Kidney stone. Coloured frontal X-ray of a male patient with a kidney stone (small purple area at centre left) in his right ureter (tube connecting the kidneys to the bladder, not seen). The spine is seen at top centre and runs down to the pelvis at lower centre. 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.