A small, hard deposit that forms in the kidneys and is often painful when passed. Kidney stones (also called renal calculi, nephrolithiasis or urolithiasis) are hard deposits made of minerals and salts that form inside your kidneys. Your kidneys remove waste and fluid from your blood to make urine. Kidney stones, or renal calculi, are masses made of crystals. A kidney stone is a hard object that is made from chemicals in the urine. Urine contains many dissolved minerals and salts.