Kidney stones (calculi) typically form when your urine becomes too concentrated, causing crystals to separate from the urine and build up inside the kidneys. Stones may be as small as a grain of sand or as large as a golf ball. Some stones pass out of the body without the need for intervention, but others can cause severe pain if they become trapped in one of your ureters (the narrow tubes that carry urine from the kidneys to the bladder). If you believe you may have a kidney stone, seek medical attention immediately. If left untreated, the issue can become significantly more sever with time.