For reasons that are not completely understood, some of the substances in urine, uric acid and calcium in particular, may crystallize within the kidney, forming rock-like particles (calculi) called stones. The medical term for this condition is nephrolithiasis or renal stone disease. Kidney stones may be as small as a grain of sand or larger than a golf ball. Depending on their composition, they may be smooth, round, jagged, spiky or asymmetrical. Most stones are yellow to brown in color, although variations in chemical composition can produce stones that are tan, gold or black.