The best stone material for outdoor use is black granite. The common igneous rock that many countertops and cemeteries use for headstones works well for laser etching murals, too. If you are considering inside use, use a large amount of black marble tiles or even tempered glass works well. The finer the grains and the more solid black, the better.
The color black works well with murals as it creates a beautiful black-and-white image. One of the most common reasons people want to behold art in black and white today is because it lends a certain timeless quality to the images.
-Eric Johnson, Kern Laser