Kidney stone. Coloured urogram (X-ray) of the abdomen of a 39 year old male patient, showing a kidney stone lodged in a ureter. The ureters join the kidneys (red, upper left and right) to the bladder (bottom centre, dark green). A dye (green) highlights the tree-shaped renal pelvis of each kidney and the long ureters. One eighth the way down the ureter at upper left is a clear space where a kidney stone (calculus) is blocking the ureter. Kidney stones form when salts in the urine precipitate and harden. Blockage of the ureter can cause pain and kidney damage. The stone can be surgically removed or broken up by lithotripsy.