A small, hard deposit that forms in the kidney and is often painful when passed.

Kidney stones are hard deposits of minerals and acid salts that stick together in urine that's not diluted. They can be painful when passing through the urinary tract.

There might be no symptoms until the stone moves around in the kidney or passes into one of the tubes that connect the kidneys and the bladder. A stone stuck in one of the tubes can cause terrible, sharp pain in the side.

Kidney stones usually don't cause lasting damage. Treatment for most stones includes taking pain relievers and drinking water. Other treatments may be needed to remove or break up larger stones.