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Low Heat Sapphires


I have a questions. I recently bought some sapphires from Sri Lanka, with their local certificates saying its unheated.

However, when I brought it back to singapore , I tested it with 2 gem labs and they concluded that the sapphires were low heat treated.

Could I check if anyone has a similar experience and know if labs specifically test for this?

Thank you in advance

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I thought I’d recently read somewhere that heating sapphires was very difficult to test, especially low heat as the gem has been heated naturally when forming. I’m sure better learned colleagues will be able to advise further.

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I defer to sapphire experts but won’t microscopic observation on the nature and type of silk (rutile) in the stone help in identifying heating? It was my understanding (at least in rubies) that lightly heated stones will sometimes show remnant silk (which more intense heat treatment will have completely removed)?

We had some clients who were sapphire shopping 2 months ago in SL. Being non-gemologists we were quite concerned that they might be over-charged. Indeed they were often being offered ‘unheated’ stones (with local certificates) at significant premiums. I sent them details of SL stones available from one of our London dealers for comparison while they were there and they decided to instead make a purchase when they were next in London (in April) - to their (admittedly inexperienced) eyes they were not being offered anything of better value on the ground in SL, plus there was the added cloud over how legitimate these certificates were. It sounds as though it was probably a better outcome for them, judging from what Elijah has found.


Hi Andy,

Unfortunately, the ones that i bought were internally flawless, so the lab had much difficulties as they didnt have any rutiles/inclusions to check for.

I had a chat with the dealer in Sri Lanka, he seemed genuinely surprised that Singporean labs made such he finding, and he was quite confident it was an unheated stone. He even guaranteed that if we were to send it to GRS, it would come back as unheated.

Another dealer i talked to said that low heat treatment was fairly common in Sri Lanka since about a decade ago.

Not sure if anyone well versed in this area would be able to offer their views?

Almost all (80%) of the sapphires in Sri Lanka are low heat treated. In fact, they don’t consider this as ‘Heat Treatment’. So, they are right and you are right! Low heat treatment is almost undetectable. So, a big high five to the lab that detected it. Would be interesting to know how they did it.

Real Heat Treatment is done in Thailand and not Sri Lanka.
raj rao - Gemz LLC (USA)

Hi Raj,

Have taken it to 2 labs in Singapore and both raised the issue of low heat treatment. They mainly looked at inclusions and a spectrometer to determine it, it definitely took longer than usual.

Hi, as there is a clear difference in heat treatment done in lab or naturally…in corundum silk is one thing that will prove it if it’s heated in lab…the rutile threads will turn into broken dots and other identification is the burst halos …which is any crystal (mineral) that melts while heating and solidifies…bears a Halo around… which is a clear indication of heat treatment in lab and not natural.

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Some Corundum can show signs of being thermally enhanced but have not been treated by man. Corundum that is brought to the surface by magma will show the same signs of thermal enhancement as those stones that have been intentionally and typically treated for color and clarity in a lab as the temperatures they have been exposed to are similar. These geographical locations are know thus the determination of origin can contribute to the determination of the process being natural or not. For instance Burma Ruby having been formed and mined from placer deposits or hard rock we know have not been naturally exposed to the temperatures of magma this broken silk or tension halos are diagnostic. Only treatments that are done after faceting and or get close to the melting point of the Corundum can the natural process be distinguished from the lab process for Corundum from known magma produced deposits. Temperatures used on rough low enough not to affect the rough or it’s inclusions are undetectable.

Hey Elijah!
My name is Chris from Navneet Gems’ marketing team.
So I have discussed this question with one of our company experts and definitely he said that in Srilanka labs are always have favorism. They will write whatever the seller wants them, because they are unknown, poorly managed, labs that don’t have enough money to afford advance equipment. GRS does not have very high credibility in the world market except for in China as they are a business friendly lab. Our company don’t like GRS so much, because Navneet is a gemologist and the GRS is too business friendly and will write terms like “pigeon blood” for deep red non pigeon blood ruby stones.
As far as low heat goes, srilankans are the master of low heat and high heat (HPHT), this is a huge thing in worldwide customers now, stones are accepted, but the buyers must know. The best lab now for coloured stones is Lotus geology, owned by Billy hugges, she is the daughter of Richard Hugges. They are genuine people who are gemologists by nature and not business friendly owners. SSEF and Gubelin are other reputable labs in Switzerland that will never make a wrong call on their stone call.

If you need really specific help, you can ask my boss directly, he will help you please visit, he also have some microscopic photos to explain low heat and high heat. But a clear understanding can be made with this image here attached.

Good luck!


Traditional Blow Pipe Treatment - Sri Lanka

Most sapphires are heated.In the industry it’s almost a given. Try using an Emersion Cell with the proper RI oil.

Hello everybody
Best quality of Sapphire & Ruby
in Sri Lanka are absolutely NOT Heated
and that is very satisfying.
Many years there
Peter from Sweden