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Blue Spinel how to tell Natural from Synthetic?

Hi everyone,

I am looking to buy two blue Spinels from a seller but I am unsure how to tell specifically if they are real or synthetic so I would like someone to give me some advice here. While its expensive, if I have to I will send them to GIA but I was hoping I could do some tests prior. Now the size of the spinels is what first leads me to believe they are synthetic because one is 40cts and the other one 45cts which is extremely large especially since they are internally flawless. One is darker than the other lighter, the lighter one has a bit of a steely blue look typical of blue spinels. It’s just the size that has me unsure. The seller told me that they would take them back if I found out they were synthetic (but I’d like to save myself the GIA fee if I could). They have been tested with the Presidium Duo detector as spinel but that doesnt tell you if they are synthetic or natural. If anyone could give me a brief overview of some tests (maybe some website links) I would really appreciate it! Thank you for your help!

Well you can check them with a spectrometer ?

Natural blue spinel have strong absorption at 632, 585, 555, 478, 458 nanometers ; synthetic cobalt blue spinel Has strong absorption at 635 ,580, 540 , and weak absorption at 478 …

A cheaper less recommended way is to use a Chelsea filter in incandescent lighting … and normally a natural blue shouldn’t glow red , but a synthetic will.


Thank you! That’s exactly the type of information I was looking for! I will definitely see what results I get from each of these methods.


I have a question if you don’t mind - you said that the red in the Chelsea filter = synthetic but some websites I have visited claim that the glowing red Chelsea filter for blue spinels actually just represent strong presence of cobalt within the spinel. One even claimed that as a defining factor of a genuine natural cobalt spinel and purposefully include it in all of their descriptions of the blue spinels as proof. Which one is it? (just so you know, I understand the first test you listed would be more definitive but I am just wondering why someone would claim it was totally authentic if that is proof that it is synthetic?). Thank you for taking the time to explain this to me, I really appreciate it!

All the best

I said the lesser recommended method it’s not definitive… I have natural blue spinel and I have synthetic blue and then studs doesnt glow but the synthetic is bright red … spectrometer is what you need forsure

Thank you for your response. I will definitely be using the spectometer then!

You could try a UV light. A synthetic stone will have brighter line in the stone called crosshatch.

My $.02:

You are talking about gems that should be selling in the range of $200k USD each (possibly MUCH more, I have never heard of blue spinel in those sizes, Graff or Harry Winston would likely sell these for $1m+++)

I would offer the owner an additional $2500 to get them certified with GIA/Gublin/SSEF and pay for rush service. GIA will also give you a nice letter about the rarity of the gems.


Jpe206, I found a video that describes that process so that is also very helpful! Thank you!


Yes, I am aware of your points, hence why I included my disbelief that it is a natural blue spinel specifically because of the size - I have never seen a spinel that size OR that intense blue as it was. After doing a bit more research I actually saw something with the presidium duo detector that is leading me to believe it is actually blue TOPAZ due to them being right above one another on the detection scale. This leads me to believe that the stone was actually a large blue topaz rather than a blue spinel. Given the size and clarity of the stone this definitely would make a LOT more sense to me. Finding ONE blue spinel that size is already remarkable but the seller actually had TWO that large which is even more rare, again making me doubt the genuine material.

Thank you all for your helpful suggestions, I will actually use all of these in my upcoming gem buys as I have been very hesitant to dump a bunch of cash unless I have a bit more tests to run to determine synthetic vs natural. I really appreciate everyone’s help.

May I ask how much the owner was selling them for?

Doesn’t sound natural in that size and if they were. And gem quality would be a collectors piece and worth a fortune. Probably a scam. You can bring them to Gem Lab at 580 Fifth ave. Ask them to test if natural or synthetic They charge $25

The first test that has always been used for these gems is refractive index - flame fusion (Verneuil) spinel has RI 1.728 versus natural 1.718 - small but very consistent. Flux grown spinels have properties that overlap the natural but you don’t usually see these in large sizes. Also, Verneuil boules will possibly show the curved growth lines called “striae” which are proof positive of synthetic and if you are lucky this will show up with a penlight through the stone in the right orientation. The likelihood of finding a 45 carat blue gem outside a museum in my opinion is slim to none, and I am leaning more towards the “none” side.