Diffusion Treated Sapphire
General Information
What is this? Diffusion treatment is a surface layer baked into the gemstone surface that imparts a false color for the stone. This should not be confused with the bulk diffusion process recently uncovered by the AGTA regarding padparadascha sapphires. This is another story for later. For now, we will concern ourselves with the surface coating of cobalt that is baked onto...and partially into....the sapphire. You should also not confuse this process with the cobalt colored green topaz. Those topaz are treated with more of a surface coating than a diffusion process. Remember that diffusion will bake the coloring element into the surface of the stone to a minimal depth...while the cobalt topaz is more of a painting process and is very fragile.
You should note that the cobalt diffusion of sapphire is not permanent. It will show through with scratches from everyday wear and tear. So while they are cheap, they are also temporary. And any sale or purchase of these stones should be accompanied by a full disclosure of the process at the time of sale.
Now......let's go see how you identify this treatment........
Now.....here comes a problem that was impossible
to photograph but you need to be very aware of this fact. When
tested for refractive index reading a cobalt surface stone will
either give you a very, very faint reading....or none at all.
Just a shadow. This should be your first indictation of a problem
if you are following normal GIA testing protocol. If you are following
YourGemologist testing protocol....this should not be a problem.
But GIA students need to be aware that the first anomoly you see
while testing diffusion treated sapphires may be a funky looking
reading. Pay attention to the signs it is giving you......something
is amiss! YourGemologist students will have used a different approach
and will probably find the true nature of this stone without the
need of that refractometer. 
There you have it! Cobalt diffusion treated sapphires should be an easy identification for anyone with a Chelsea filter and/or a spectroscope. Remember, you are not trying to prove what it is....you are looking to prove what it cannot be in hopes that it will leave you with only one possiblity. On this case, there is only one possibility this could be. Cobalt Diffusion Treated Sapphire. Well done to all. YourGemologist
With greatest appreciation to:
Synthetic Imitation & Treated Gemstones by Michael O'Donoghue, Butterworth Heinemann, London, 1997, and
Gems Made by Man, Kurt Nassau, Gemological Institute of America, Santa Monica, 1980
Without whose excellent books
on this subject these web pages would not be possible. YourGemologist.com
Please note: All photographs and all text on this page are © Copyright 2002 by YourGemologist.com
I prefer not to have to mess up the photographs with a bunch of copyright notices so please respect the copyright for the sake of everyone else who visits this site.