Most people who have had a kidney stone (called renal lithiasis) attack will tell you it is a very painful experience. The stones are usually small, hard deposits that form inside the kidneys and are made of minerals and acid salts. They often affect many places along the urinary tract, from the kidneys to the bladder, and can form in both or a single kidney. They seem to form when the urine becomes concentrated and/or when the urine contains more crystal-forming substances, such as calcium or uric acid, than can be diluted by the fluid in your urine. The stones are most commonly classified as calcium, struvite, uric acid, and cystine stones—all of which form for various reasons.