diffused sapphireDiffusion Treated Sapphire


Consumer 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


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