X-rays are a quick, painless test that takes pictures of structures inside the body — especially bones. X-ray beams pass through the body, and they are absorbed in different amounts depending on the density of the material they pass through. Dense organs, such as bones and metals, appear white on X-rays. Air in the lungs appears black. Fat and muscle appear gray. For some types of X-rays, a contrast material — such as iodine or barium — is introduced into the body to provide more detail on the images.