Kidney stones are hard deposits that can develop in your kidneys. Your kidneys are essential organs that filter waste products and excess water from your blood and produce urine. Sometimes, certain substances in your urine, such as calcium, oxalate, or uric acid, can build up and form crystals. Over time, these crystals can combine and grow larger, forming kidney stones. These stones can vary in size from tiny grains to pebble-like rocks and can cause severe pain when they move or block the urinary tract. Treatment options for kidney stones can include drinking plenty of water to help them pass naturally or, in more severe cases, medical intervention may be required.