A kidney stone is a solid mass made up of tiny crystals. One or more stones can be in the kidney or ureter at the same time. See also: Cystinuria Causes, incidence, and risk factors Kidney stones are common. Some types run in families. They often occur in premature infants. There are different types of kidney stones. The exact cause depends on the type of stone. Stones can form when urine contains too much of certain substances. These substances can create small crystals that become stones. The stones take weeks or months to form. Calcium stones are most common. They are more common in men between age 20 – 30. Calcium can combine with other substances, such as oxalate (the most common substance), phosphate, or carbonate, to form the stone. Oxalate is present in certain foods such as spinach. It’s also found in vitamin C supplements. Diseases of the small intestine increase your risk of these stones. Cystine stones can form in people who have cystinuria. This disorder runs in families and affects both men and women. Struvite stones are mostly found in women who have a urinary tract infection. These stones can grow very large and can block the kidney, ureter, or bladder. Uric acid stones are more common in men than in women. They can occur with gout or chemotherapy. Other substances also can form stones, including the medications, acyclovir, indinavir, and triamterene. The biggest risk factor for kidney stones is not drinking enough fluids. Kidney stones are more likely to occur if you make less than 1 liter of urine a day. That’s slightly more than a quart.