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.

Some stones stay within the kidney, where they often produce no symptoms. Other stones may break loose and travel down the urinary tract. The smallest, smoothest stones may pass out of the body with little resistance and minimal discomfort to the patient. Larger, irregularly shaped stones cause intense pain. They also can become lodged in the ureter, bladder or urethra, the tube that carries urine from the bladder out of the body.

In addition to causing severe pain, a lodged stone can block the flow of urine, causing wastes to back up into the kidneys. Such a condition must be corrected swiftly, either by surgically removing the blocking stone or by nonsurgical medical procedures that break it up and allow it to pass naturally out of the body. If not, serious kidney damage and related medical problems can result.