Substances in urine (uric acid and calcium in particular) can crystallize within the kidney and form rocklike 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 yellow or brown in color, but variations in chemical composition can produce stones that are tan, gold or black.

Some stones stay in the kidney and produce no symptoms. Other stones break loose and travel down the urinary tract. The smallest, smoothest stones may pass out of the body with little resistance or discomfort. Large, irregularly shaped stones can become lodged in a ureter (the tube that carries urine from the kidney to the bladder), the bladder, or the urethra (the tube that carries urine from the bladder out of the body) and cause intense pain.

A lodged stone can block the flow of urine, causing pressure to build up in the kidneys. Such a condition must be corrected swiftly to prevent serious kidney damage and other medical problems. The stone can be removed surgically or can be broken up and then passed naturally out of the body.

Different chemical imbalances in the urine produce stones with different chemical compositions and shapes. The four most common types of stones are made of calcium, struvite, uric acid, and cystine.

Calcium Stones About 70% to 80% of all kidney stones are composed of hard crystals of calcium oxalate, insoluble phosphate salt, or both. They occur in patients with hypercalciuria (excessive calcium in the urine).

In about 40% of patients who develop calcium stones, this build up is caused by an inherited metabolic disorder. In rare cases, a tumor on the parathyroid gland may trigger an overproduction of parathyroid hormone, the chemical that regulates calcium metabolism. Intestinal disease, excessive amounts of vitamin A or D, and a diet too high in purine (associated with meat, fish, and poultry consumption) also can cause hypercalciuria.

Calcium oxalate stones are commonly associated with vitamin B deficiency or an excess of vitamin C in the diet.

Struvite Stones Struvite stones (also called infection stones) account for up to 20% of all kidney stones. This often occurs in patients who develop urinary tract infections. Struvite stones are more common in women because they generally suffer more urinary tract infections than men. This type of kidney stone typically develops as a jagged or branched structure called a "staghorn."

Cystine Stones Cystine is an amino acid found in nerves, muscles and other body tissues. Cystinuria, a rare genetic defect, can cause excessive cystine build-up in the urine and lead to the development of cystine stones. This type of stone occurs in about 1% to 2% of patients with kidney stone disease. The condition often is hereditary.

Uric Acid Stones Uric acid is a by-product produced by the body as it breaks down protein that is normally flushed out by the kidneys in urine. Some people, particularly men, build up excessive uric acid concentration in their kidneys or joints. In the joints, this can cause gout, an inherited disorder with painful arthritic symptoms. If uric acid builds up in the kidneys, uric acid stones may form.

An estimated 5% to 13% of patients with kidney stone disease develop uric acid stones. Genetics may be a factor in uric acid stone propensity. Patients prone to developing uric acid stones are advised to reduce their consumption of high-protein foods, especially meat.