Kidney stone. Artwork of a large kidney stone. 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. Thee are many different causes , including dehydration, infection and metabolic disorders. Most stones are small enough to be passed in the urine, but large stones like this need to be treated. The stones may be remove by surgery or non-invasive means. These include extracorporeal shock wave lithotripsy, which focuses shock waves on the stones, causing them to shatter.