X-rays are composed by X-radiation, which is a form of electromagnetic radiation.

To produce an X-ray image of bones, short X-ray pulses illuminate the body with radiographic film placed behind it. With photoelectric processes, bones are able to absorb much more X-ray photons because of their higher electron density comparing to soft tissues. If there is a fraction on the bones, you will be able to see that on the X-ray image because the fraction part won't be able to absorb any X-ray photons, which leaves the image blank.

To find out more about X-ray, check here.