A kidney stone develops from crystals that separate from urine and build up on the inner surfaces of the kidney. Normally, urine contains chemicals that prevent or inhibit the crystals from forming. However, in some people, stones still form. Crystals that remain small enough will travel through the urinary tract and pass out of the body in the urine without even being noticed.
- Calcium stones. Calcium stones are the most common type of stones. Calcium is a normal part of a healthy diet and is used by bones and muscles. Calcium not used by the body goes to the kidneys where it is normally flushed out with the rest of the urine. In some people, however, the calcium that stays behind joins with other waste products to form a stone.
- Struvite stones. Struvite stones are a type of stone that contains the mineral magnesium and the waste product ammonia. It may form after an infection in the urinary system.
- Uric acid stones. Uric acid stones may form when there is too much acid in the urine.
- Cystine stones. Cystine, one of the building blocks that make up muscles, nerves, and other parts of the body, can build up in the urine and form a stone. Cystine stones are rare. The disease that causes cystine stones (cystinosis) runs in families.