Kidney stones. CT (computed tomography) scan of the abdomen of a 60-year-old with stones (white, upper left) in the right kidney. Kidney stones are hard lumps (calculi) that form when mineral salts, often containing calcium, accumulate inside the kidney. Causes of this deposition can include dehydration, infection and metabolic disorders. Although most kidney stones are small and can be passed in the urine, if they grow too large they can block the flow of urine from the kidney and cause extreme pain. They can be removed by surgery or by shock wave lithotripsy, which shatters the stones into small particles so that they can be expelled naturally by the body.