Kidney stone in the ureter. Illustration of a sectioned kidney, showing a kidney stone (calculus, lower right) in its ureter. A kidney stone is formed by the precipitation of the mineral salt calcium oxalate from the urine. The resulting hard deposit may cause severe pain, especially when it passes down the urinary tract. A small stone may be passed spontaneously. A large stone may block the flow of urine and is removed surgically or by using sound waves, applied externally, which break it up. Also shown in this illustration are the renal artery (red) and renal vein (blue), the renal pyramids (darker red), and the renal pelvis (where urine drains into the ureter).