YAG laser crystal. A YAG (Yttrium-Aluminium Garnet) crystal synthesized for use in a laser (light amplification by stimulated emission of radiation"). A radiation source is used to stimulate unstable atoms in the crystal to emit light at the same frequency as the source. This produces a beam of intense monochromatic coherent radiation, red light in the case of YAG. YAG laser crystals are often doped with neodymium (Nd- YAG) or thulium (Tm-YAG) and are used for cutting metal and as surgical tools."