Kidney stones. Coloured frontal X-ray of the pelvis of a male patient with kidney stones. 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. Here, the patient's bladder (yellow) has enlarged, indicating that a kidney stone may have moved to the urethra (the tube through which urine leaves the bladder) and blocked it, causing the bladder to overfill. Most stones are small enough to be passed, but larger stones, such as this one, 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.