A kidney stone is a hard mineral deposit that forms in the kidneys. It occurs when certain substances in the urine, like calcium, oxalate, and uric acid, become concentrated and crystallize. These stones can vary in size and may cause pain when passing through the urinary tract. Symptoms of a kidney stone can include severe pain in the back or side, blood in the urine, and frequent urination. Treatment options range from drinking plenty of water to help pass the stone, to medical procedures for larger stones that cannot be passed naturally.