Staghorn kidney stone. X-ray of the abdomen of a patient with a large staghorn stone (white, upper left) in the right kidney. Kidney stones are formed when mineral salts, most often involving calcium, accumulate on the inner surfaces of the kidney. As they grow they may block the flow of urine out of the kidney, causing extreme pain. There are many different causes, including dehydration, infection and metabolic disorders. Most stones are small enough to be passed in the urine, but larger stones like this need to be treated. The stones may be removed by surgery or non-invasive means. These include extracorporeal shock wave lithotripsy, which focuses shock waves on the stones, causing them to shatter.