Of the four main types of urinary stones, calcium stones are the most common. Almost 95% of all kidney stones are clacium stones. They occur when there is too much calcium in the urine. Defective kidney function may allow too much calcium in the urine, or excessive calcium may be absorbed from the stomach and intestines. Some calcium stones are caused by an excess of a chemical called oxalate, present in many foods, which binds easily with calcium to form a stone. The risk of calcium stone formation is increased if you have certain medical diseases; for example, hyperparathyroidism and inflammatory bowel disease.

A second type of kidney stone is made of uric acid. These stones are much less common than calcium stones. Uric acid stones occur because you have too much uric acid in your urine. They might occur if you have become dehydrated; for example, during strenuous exercise on a hot day or perhaps during an illness. Uric acid stones are common in people who have gout, a disease that causes high uric acid levels in the blood.

A third type, struvite stones, are not quite so well understood. It is thought that these stones form as a result of an interaction between protein-breakdown products and infection-causing bacteria in the urine.

Finally, a rare type of kidney stone is a cystine stone. It occurs if you have the genetic disease called cystinuria. This disease results from a birth defect that causes the kidney to allow too much cystine into the urine. This type of stone formation is almost always diagnosed during childhood.