Kidney stones (also known as renal calculi, nephrolithiasis, or urolithiasis) are hard deposits of minerals and salts that form within the kidney. As urine passes through the kidneys, it can become extremely concentrated. When urine becomes too concentrated, calcium, uric acid salts, and other chemicals dissolved in it crystallise to form a kidney stone, also known as renal calculus. Typically, calculus is the size of a little pebble.