Kidney stone. X-ray of a stone (white, upper right) 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 deposit may cause severe pain, especially when it passes down the urinary tract. A small stone may be passed out of the body spontaneously, but a large stone 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 stone.