Kidney stone is a hard object that forms in the kidneys. Kidney stones range in size from microscopic to about as large as a golf ball. They occur primarily in men and can cause intense pain if they become stuck in the urinary tract. Most kidney stones consist of calcium salts. In many cases, doctors cannot determine why the stone forms. Some people who develop kidney stones absorb an unusually high amount of calcium from their diet. Excess calcium is eliminated in the urine. However, some of the calcium may crystallize before it leaves the body, forming a stone.