Bladder stone. Coloured scanning electron micrograph (SEM) of a bladder stone from a dog, showing the crystalline structure. Bladder stones, like kidney stones, are usually formed by the precipitation of the mineral salt calcium oxalate from urine. The resulting hard, round stones (calculi) may cause severe pain, especially as they pass down the urinary tract. Large stones, which may reach several centimetres in diameter, may need to be surgically removed or broken down using ultrasound therapy. Magnification: x10 when printed at 10 centimetres wide.