Kidney stones. Gross specimen of part of a 56- year-old patient's kidney with large kidney stones (upper right and lower centre). 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 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.