Kidney stones. X-ray of numerous stones (centre) in the kidney of a patient. Such stones are usually formed by the precipitation of the mineral salt calcium oxalate from the urine. The resulting hard, spherical deposits may cause severe pain, especially when they pass down the urinary tract. Small stones may be passed out of the body spontaneously, but large stones may block the flow of urine and must be removed. This may be done surgically or by using sound waves, applied externally, which break up the stones. The spine runs down far left.