Diffusion is also popular these days, but it's almost as notorious as radiation, as the results aren't guaranteed or permanent. Diffusion introduces coloring agents onto the top-most layer of the gemstone with the help of high-temperature heating for prolonged periods of time. It only colors the surface of the stone, and even that may fade with time. Any chip or crack on the surface will show through, though the treatment can be administered again and the fault re-colored. This is found in sapphires, rubies and the artificial production of green topaz. Sapphires are the most commonly treated this way, producing glorious shades of blue, from the pale turquoise to the richest cobalt blues. Don't be confused if someone admits that the gem has been through “deep diffusion”, as this only means that the colored layer is slightly deeper than a regularly diffused stone.