Substances in the urine (calcium, oxalate, phosphate and uric acid, in particular) can crystallize within the kidney and form 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. They may be smooth, round, jagged, spiky or asymmetrical depending on their composition. Most stones are yellowish to brownish-black in color, but variations in chemical composition can produce stones that are tan, gold or black.