Scanning electron micrograph of a kidney stone (nephrolithiasis). Kidney stones form when salts, minerals and chemicals in the urine (for example calcium, oxalate and uric acid) crystallize and solidify. Small kidney stones are often passed naturally but larger stones can sometimes get lodged in the kidney or other parts of the urinary tract. Size of stone is 2 mm. Supplied by Kevin Mackenzie, courtesy of Wellcome Images.