Kidney stones (also called urinary stones or renal calculi) are mineral deposits that are formed in the kidneys, the two bean-shaped, fist-sized organs located on either side of the spine beneath the surface of your mid-back muscles. As blood flows through the kidneys, these organs filter out the waste products that the blood has carried away from the active cells in your body.
Additional impurities and excess minerals are also filtered. The cleansed blood continues on its path through the body, and the trapped impurities stay behind, eventually mixing with water and passing out as urine. Sometimes, though, the filtered minerals build up in the kidneys where they can crystallize and develop into stones. Although formed initially in the kidney, these stones can migrate and be found anywhere along the urinary tract. The presence of stones in the kidney is not always associated with pain, but if the stones move, the pain can be excruciating.