Good day,
I saw a gem on sale under the label of “greenish blue spinel from Burma”.
As I am not that big spinel expert, I would like to ask your opinion, attaching the photo. I have never seen such color spinel, looks more like tourmaline to me.
Oleksandr,
It is definitely a beautiful green. I personally haven’t seen a spinel with that color hue.
There are several tests you can do to help distinguish spinel vs tourmaline. Unfortunately, a picture alone cannot determine its nature.
Take a look at these articles on the two types of gemstones.
There are features and properties unique from each other. A refractometer and polariscope would be very helpful to have. A quick test though would be Specific Gravity.
Specific Gravity, for these two gems, however, overlap each other.
There is only one mineral in the spinel subgroup that is “green”: Gahnite
tourmaline SG: 2.82 - 3.9
spinel SG: 3.5 - 3.9
Gahnite is 4.0 - 4.62 SG. If you measure SG in this range you can probably rule out tourmaline.
Admittedly, it does look more like a tourmaline… it is difficult to identify with just an image.
On a side note, I have a small parcel of rough that I bought a while back as "Gahnite, but it is a deep blue color. I most-likely bought gahnospinel. Had I done some research before buying this (like reading the spinel article) 
…I would have been better informed. I still have to take measurements on the material to verify what I have, though. 
Cheers!
-Troy
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Thanks Troy. The thing is that it is on sale online. If I have it in my hands I would just order gemmological examination, but this is not the case
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Well…had I read your original post more closely, the situation would have been more obvious to me…sorry!
So there is a way to estimate the material weight based on the dimensions (if posted). You can always ask the seller for those.
Calculate the dimensional volume of the stone and compare it to the known density for that material.
This is a very rudimentary estimate.
I cheat a little by using either GemCAD or Gem Cut Studio to estimate a stone’s weight and material based on basic facet cuts and shapes using the stone’s dimensions.
If the numbers give me an approximation…I have better confidence on purchasing a particular stone.
It definitely is not “all seeing” but it can give you an idea about the stone in question.
I can do a quick estimate for you if you are not familiar with either software suite.
Cheers!
-Troy
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I’m sorry to tell you that you guys are missing the obvious. Tourmaline is dichroic and doubly refractive (two refractive indices) and spinel is not dichroic and is singly refractive. A deep colored stone like that, if it is tourmaline, would show one color down the C axis and another color down the A-B axes. If it is dichroic, you can see that by holding it up to the light and turning it. If it’s dichroic, it isn’t spinel. You could use a dichroscope, too, but that’s more for things like pale aqua, which you will have trouble seeing the two colors in, but the dichroscope will show them. The dichroscope will show the two very different colors, but I don’t think anyone will miss them in a deeply colored tourmaline. In fact, I have a very bright 6 year old grandson and I have a piece of tourmaline that isn’t worth cutting which I’m going to take to him…he loves rocks and crystals and is reading at a second grade level already, so he’ll learn a new word, “dichroic,” and then I have a piece of iolite which I’ll also give him, which show three colors, “trichroic.” It show blue, yellow and colorless, more or less, but definitely three distinct colors. The tourmaline is a light pink down the C axis and a very washed out faint pink on the A and B. Much more subtle than your example, but still easy for a bright 6 year old to see.
My copy of Webster’s Gems: Their Sources, Descriptions and Identification says there is a “subdued blue-green” spinel and that a “very dark” green spinel also occurs, but they are, as you know, not typical and pretty rare. So if this stone, if you had it in hand, turns out to be singly refractive and NOT dichroic, it could be spinel. If you show it isn’t tourmaline, you can proceed to get an SG (just need a scale and a cup of water) or an RI (refractometer). That will narrow it down some. I haven’t checked the UV response or the possible magnetic response, which would be other easy tests. If you suspect it is spinel, then looking with a microscope for inclusions to determine synthetic vs natural would also be important. The RI of synthetic is usually different from natural, but there can be an overlap, hence the focus on inclusions. There is a blue-green spinel (google “synthetic blue green spinel images”) but I think the shade of blue green is usually a little different from what you picture…however, IDK how many labs make this synthetic or whether a deeper color couldn’t have been made in the past, etc.
Please read some in a gemology text or take a course here or look for Barbara Smigel’s free gemology course. It is really dangerous to be buying stuff for much money if you don’t know the basics. I hope this is helpful and excuse my frankness. If you can’t pretty positively ID what you are buying and don’t have a gemologist to consult, you are at the mercy of dealers who may fake stuff or who may get parcels which they don’t know are salted with synthetics or glass or whatever. If you sell cut stones, you better know what you are selling is what you claim it is, because if you sell something and ID it wrong, everyone will assume that you can’t be trusted and probably did it on purpose. News travels fast in the stone world and lots of people know each other and it only take one or two sales gone bad to cook your reputation, maybe not with $50 quartz, but definitely with high dollar stones. -royjohn
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