The following is a brief and exploratory study of a 3.87 carat yellow tourmaline I purchased from www.ajsgems.com Gems some time ago. The methodologies involved are non destructive and include microscopic testing as well as some imagery coupled with long wave UV testing. I thought I would share at least some of the more interesting pictures I obtained, and a few artifacts that threw me for a loop-but in a good way.
Yellow tourmaline is more or less a collectors stone, and anything particularly of gem quality of a carat or better is considered a collector size stone. The rough is often incredibly included, so there is little yield as a result. This study is meant to be a brief version of a more complete study of like kind that may or may not be published later on. These are simply the highlights and some things that are of interest.
I made the mistake of wiping the stone with a lotion containing tissue after rinsing it, hence why I could never get a good face up shot. It’s all dirty as a result, but it is remarkably clean for a yellow tourmaline-especially of this size. The polariscope without the polars crossed shows a slight pleochroism, but under various lighting conditions I have noticed a more obvious pleochroism within the stone. I can also tell you that it is uniaxial negative with a chronoscope, which is consistent with yellow tourmaline by pretty much any account I could find.
This is under 32x or thereabouts. There is no evidence of growth rings, but you can being to see some very tiny crumb like inclusions. This stone while shallow has now eye visible window but it shows under the microscope. The background was a white background yet we see the blue hue behind it. The exterior of the FOV was yellow, so when you see blue on the inside, yellow out, what does that yell you? Negative optic character. I had no qualms finding a uniaxial sign and it’s nearly a text book case of it. The significance here is to illustrate the lack of fracture type inclusions that you typically see in rough yellow tourmaline, and the fact that it is in fact a natural stone.
We can see a very minimal absorption of UV spectra in these images. UV has relatively no effect on this specimen unlike others. Then again yellow tourmalines are not considered to be elbaites like other specimens of tourmalines studied so far. These simply have the basic chemical structure of tourmaline in general and are thus included in that family.
If I am not mistaken, under 39x, I observed these partial pink curves in the area of the left crown near the bottom. I’ve seen things like this under the loupe and with the other pink tourmaline specimen that I studied. The difference is with the last specimen, it was under diffused polarized light, and in another it was loupe visible from some smaller pink tourmalines. In either case, they appear to give a zoning appearance within pink tourmalines originating from Nigeria. This yellow was disclosed as being from Idaban in particular. This by no means is something to be considered to be a sign of synthetics as some of these artifacts can appear to be curvilinear. However, I’ll illustrate these artifacts later on. These are quite interesting.
Here is the same area near 55x. We don’t see these same artifacts, but we can see these tiny bread crumb inclusions littered throughout the specimen. To me, this is a tell tale sign of evidence supporting the origin. These appear to be mica inclusions, and these are consistent with any tourmaline I’ve ever seen from Nigeria so far or have heard about. I was trying to see the pink artifacts better as they didn’t match the host crystal at all, so they were of interest.
Though the light interferes, this is at 205x. We can see some solid and even rain like inclusions. There is a rogue shaped inclusion that is more needle shaped, but this thing is littered with very small inclusions what I believe to be mica’s of some sort. I could be wrong, but they are inclusions consistent with Nigerian tourmalines. You can see some slightly rounded edges on one or 2 artifacts which would suggest heat. However, that’s inconclusive to say it has or hasn’t been heated other than whatever mother nature has done. This stone was disclosed as being unheated, and there are some irregular shapes that seem to suggest that and a lack of decomposition in these artifacts overall. These mica like inclusions look like other in other types of gems that have been identified by other sources as artifacts of unheated specimens.
This is an alternate view of the same stone at 205 x. These are irregular shaped crystals. They seem to be in clusters, so between the shape and arrangement, it is possible that the above needle like crystal may have been a negative form a naturally healed fracture-which can happen. It is possible that some volcanic event heated the crystal, and then later on it continued to grow. These are not rounded decomposed chunks.
This is at 217x, and we can see a clear double refraction of mica’s. This behavior is consistent with other tourmalines I’ve studied.
This is the pavilion at 25x, and perhaps a better illustration of this lovely golden honey color. Be it a purer yellow (which halogen light will make this stone to appear as), or a honey color, this is a rare color in tourmaline. They are starting to become popular, and I’ve seen some appraisals on some stones of this and other colors that are quite surprising. You can however see certain bread crumb inclusions in here at best.
I decided to see if I could get a better idea of the pleochroic effect I originally discovered when I first obtained the stone, and UV light explains this. I performed the same tests with UV-A with the pavilion facing upwards. Check this out under the same magnification.
This is with UV-A on. You see how things appear.
Shut it off, and there is that green. I therefore theorize that certain light sources with UV will cause a green undertone to appear as I originally observed with the unaided eye. It is comparable in appearance to what one might see in natural fancy yellow diamonds under observation with a grading lamp and loupe or microscope if not the naked eye. The green tones are the comparable. This green can be more or less apparent in the presence of a known light source available to me containing both long and short wave UV in the spectrum. Under halogen lighting, this is absent as well as any light source in which there is a lack of UV. Under halogen lighting sources, there is more of an orange hue than a green hue with fluorescent lamps.
Take note of 2 things:
1. Note the left pink zoning that exists as illustrated earlier. These appear in pink elbaites from Nigeria, so what are they doing in this yellow? This is at 75x from the pavilion view and otherwise invisible.
2. If you look towards the bottom right, you can see the outline of something like a slightly distorted hexagonal shaped crystal. This would suggest perhaps some minimal decomposition from heat, but this doesn’t occur that minimally (as far as I know) from standard heat treatment. The camera might not have this focused right giving the illusion of decomposition when in fact it could be euheadral. There is a line near the middle that appears in tact, and some mica artifact over this crystal. Barring focus limitations, this is a good indicator (potentially) of the absence of thermal enhancements for this material.
Now, on to the pink mystery. This to me I found really intriguing and it appears to be metamorphic or some isolated chemical reaction perhaps from when the crystal formed. I can only speculate as to what this material is, but it would explain the zoning appearance in some other specimens from Nigeria.
This was obtained at 190x. You can see some pitting, and some lovely pink and peach undertones. We can see some pitting here. It’s as if these things are opaque shells that live within the host matrix as solid inclusions. They have a slight wave texture to them, and that’s what we sometimes see that causes zoning in some tourmalines. I was not expecting this, but these are very small in this specimen. I wanted to get the detail though as best as I could since there is nothing I can find like it.
This is an alternate view of the same as above.
When we reach 205x, we can see some more pitting but not as much detail. These are like microscopic hemispherical shells.
This is around 45x, and this is what appears to be perhaps a naturally healed area. You can see some clear like structures that have an absence of any filling substance. There is evidence here of clear tubes consistent with what we know of in natural tourmaline (especially from Nigeria).
This is at 55x from the pavilion. If you notice the bright center part, we see something with a very slight double refraction and quite reflective. It doesn’t immediately appear to match the facet pattern from the crown. This is possibly a crystal. Sometimes that’s how these things can appear in some specimens, but it isn’t always the case. However, the fact that it doesn’t appear to match the facet pattern raises the question if this is the reflection of a crystal that is in tact.
These are simply some single phase inclusions near 65-70x from the pavilion that live in the stone. There is also some lint there too.
There is evidence of double refraction, so that is consistent with tourmaline in general. The optic sign is decidedly different from that of quartz in that it lacks the rainbow of colors you generally see, and it shows neither a left or right hand bias. The color of this under a grading lamp is considerably stronger than that of yellow quartz. While there may not be a smoking gun persay, there is evidence of some thermal event. Typically with thermal enhancements, we would see much smoother and more rounded inclusions as opposed to something that appears to be partially rounded with some irregularities that come afterwards. So based on appearance the evidence suggests any thermal event was a naturally occurring phenomena.
-joe
Now, when I got to 62x (?) beneath the crown, I saw that pink something again that caught my eye and I just now observed something else tell tale.
