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GeneralUsambara effect - tourmaline
15th Jul 2013 23:37 UTCBarry Miller
( http://www.nordskip.com/usambara.html ). I wonder if someone on Mindat is sufficiently familiar with the phenomenon to explain why it occurs and perhaps someone may have had better luck in taking such photos.
17th Jul 2013 15:29 UTCOwen Lewis
Usumbara effect is one of the colour change phenomena that can result from a transition metal in a ligand field. Another and better known example of this type of colour change is the alexandrite effect (best known for occurring in suitably doped chrysoberyl).
However, the alexandrite effect requires a change in the wavelength of the illuminating radiation for it to be seen. The usumbara effect requires a change in the depth of the colour-causing material (which does not have to be tourmaline, though it was first observed in that stone). The Usumbara colour change is not always red/green but can be between two of several different colour states, depending on the chromophore and thickness of slices. Usumbara effect has been convincingly demonstrated in transparent plastic, as well as in crystalline material.
The main tricks to create an usumbara effect are (assuming tourmaline) as follows. The tourmaline pieces should be slices, transparent , monochromatic and with the chromophore of a known transition metal. Cr or V are good choices, So Chrome Uvite? To get the best effect, the thickness of the slices needs to be calculated (or found by trial and error). Thicknesses are in the range 1-12 mm and the selection can be made off a graph on p96 of Nassau's 'The Physics and Chemistry of Color. The Fifteen Causes of Color', where the necessary supporting explanations can also be found.
For a classic photo, user the following arrangement. Cut one of the slices approximately 50%+ greater than the other. Photo the two slices side by side, showing that both are the identical shade of green. Move the smaller slice, stack in top of the larger, and take a second photo. The uncovered perimeter area of the larger and lower slice will be seen as the same green as in the first photo - but the entire area of the smaller clice will now be seen as a bright red, because of the longer transmission path of light through the combined slices.
Basically, if you can display an usumbara effect, there is no problem in image capture. The trick is in getting the effect to show in the first place.
Good luck!
17th Jul 2013 17:31 UTCAlfredo Petrov Manager
17th Jul 2013 18:36 UTCOwen Lewis
Yes, the colour change does look like it might be depth-related.but one would been to manipulate and analyse the light from the stones to have any certainty.
There's another, more subtle, colour perception effect that, only today, I became aware of. This deals specifically with blue-violet shift in colour perception and whether it might be depth related. I'm sure that my friend, Michael Hing if DeBeers UK , will not mind if I quote him here:
'I've noticed that, when people write about the Purkinje and Bezold-Brucke colour-perception effects, they tend to use blue-violet as an example when discussing how the saturation and hue position changes as the luminance increases or decreases. I always assumed that they were just using this particular colour as a hypothetical example but, come to think of it, maybe the blue-violet hues are particularly susceptible to this kind of effect? That would explain a lot.'
If you are as unfamiliar as I am with these effects, I'll be happy to find out more and share with you in due course
17th Jul 2013 22:59 UTCBarry Miller
17th Jul 2013 23:34 UTCChester S. Lemanski, Jr.
Would you consider putting the data you presented in this thread, perhaps with some photos (if available into a Mindat article?? That would be super. The article could then be referenced for any species which displays the phenomenon.
Chet Lemanski
18th Jul 2013 01:43 UTCAlfredo Petrov Manager
18th Jul 2013 02:20 UTCMike Keim
I have had success simply putting a penlight beneath the crystal and shining the light up through the bottom.
Mike
18th Jul 2013 13:54 UTCOwen Lewis
It's hard to be sure without standing at your shoulder and watching but what you describe does not sound like usambara effect to me but more like dichroism. With pleochroism, the colour you see is determined by the crystal axis you view along. Do you own a dichroscope? This small and simple instrument is the best way of confirming whether a colour views is pleochroic or not. Failing all else use a lense from a good paor of Polaroid sunglasses. Only with some gem minerals in which the pleochroic effect is very string is it easily confirmed by the naked eye. It can be weak or even non-existent,
Another way of saying this is that transmitted colour changes with the speed at which white light travels through a crystal. No point in writing this up; it's all in the text books, but if you need more, just say.
Tourmaline is, optically, a uniaxial crystal. If you view it at right angles to its c-axis through a polariser, by rotating laterally either the polariser or the stone around this viewing axis you can watch the colour change from one to a second colour and, eventually, it you continue rotation, the colour will change back again. In uniaxial crystals, there will always be just two colours unless the tourmaline is colourless, in which case no colour will be seen in any position. Depending on the composition (and hence the colour) of the piece, there may be a color change or a change in colour tone in of the same colour. In coloured tormaline a colour change is common.
Now, re-orient the stone and view it if down its C axis and through a smooth pinacoidal termination. rotate the stone laterally around the c-axis. No colour change will be observed.
This, I guess accounts for what you are seeing.
In tourmaline, or in the 5-8 gem species of it anyway, where there is colour, some pleochroist colour change is always going to be present - unless viewed down the c-axis. It gets worse. alexandrite effect colour change and usambara effect colour change may both also be present. Thus, to be sure that what you observe is an usambara effect, you must first eliminate any possibility that what you see is caused by the other two.
Elimination:
1. Pleochroism. Present in all transparent coloured tourmaline. Eliminate by viewing down the c-axis.
2. Alexandrite effect. Not strong in tourmaline anyway, even when present. Work in a darkened room where all the light comes from a single known source. In this way, no alexandrite colour change can occur.
In conclusion the usambara effect is rare. Despite well over a century of intensive study of the interplay of light and gemstones, it was not identifiec and documented until 1997. You need gem grade tourmaline with an appropriate chromophore. You then need to prepare a small range of slices that you can stack up on each other.
Testing for usumbara effect is painstaking. The odds against spotting it 'in the wild' - or even in a faceted gemstone are long. Mike O'Donoughue, a leading British gem researcher and writer of gemmology reference texts has said that the occurence of usumbara effect in a faceted gem 'is likely to defy all comprehension'
The range of gemstones in which usambara effect may be found remains undetermined. It has been positively identified in some E.African garnets (a stone without no pleochroic effect)
There is a heap big row in the gemmological world that has been running on, nastily, for years, as to whether certain colour alterations in some gemstones (including tourmaline) are natural or result from an unknown method of treatment. Whilst it seems reasonably easy to discount the method of treatment proposed by the one camp, it seems likely that the part - if any - that usambara effect has in the effects observed has never been examined and that enquiry into this possible explanation may only now be beginning.
Re. light source. What Alfredo says is quite correct - but you need not worry about it for our present considerations. The spectrum of sunlight has more holes in it than a swiss cheese but we all use that happily enough for many tasks - and that changes with season, weather, latitude and time of day too - never mind the 11 year sunspot cycle! :-D An LED source is fine for the relatively crude qualitative testing we are discussing - but not for diamond grading. A white LED would be my personal choice for the work we discuss from my 12+ 'golf-bag' range of light sources, ranging from a hardware store flashlight to an $xxxx xenon arc lamp.
18th Jul 2013 14:14 UTCOwen Lewis
There is more info in my second post to Barry that may give a slightly more full picture of some of the difficulties here.
In short nothing is quite as cut and dried as a mineralogist would find useful in field testing. The effect has been properly investigated only in tourmaline and green grossular. It's presence in other gem minerals is speculative but more probable than not. There may well be more work done in another mineral in the next year or so and it would be my pleasure to write up that work when completed.
The photo of which I have a copy is copyright and the property of the Mineralogical Museum of Oslo, Norway. Photo by Per Aas. The two pieces they used to make the picture are a slice of green tourmaline at 90 deg to the c-axis and a green cut and polished (square cut) green tourmaline (I assume cut from the same rough). Perhaps one of our Norwegian members could persuade them to donate a high definition copy to Mindat with full details of the material used.?
18th Jul 2013 14:35 UTCOwen Lewis
You *sure* you have any colour change effect of any sort there and not just a tourmaline of mixed colour? From Mada?
May we see a compative shot to showing the 'colour change'?
Sure is a pretty piece though :-)
18th Jul 2013 15:19 UTCMike Keim
The specimen in the previous photo is a dravite from the Umba Valley in Tanzania, it is already sold but here are photos of another one. When you shine a light on the top, you can see that the crystal is clearly green in the small reflections. When you backlight it, passing light through the entire crystal, the thicker areas turn red. I believe it was from crystals from this locality that the effect was first described.
18th Jul 2013 15:42 UTCBecky Coulson 🌟 Expert
18th Jul 2013 16:23 UTCOwen Lewis
1. You at showing a strong 'tourmaline' pink. This is not one of the colours predicted for usumbara effect and is commonly produced where Mn trace is the chromophore. Bi-coloured/multicoloured stones are not suited to proving usambara effect. I'm not saying it can't be dome only that it has not been done and you are making life unnecessarily difficult for your self by trying.
2. You are rotating the stone noticably between the two shots in both the horizontal and the vertical plane. This means that at least one of the two shots is not taken along the c-axis and that some pleochroic colour alteration must be be occurring.
3. The largely black appearance of the specimen in the first pic suggest that this specimen exhibits a 'closed' c-axis. An unhelpul and common condition in gem tourmaline that reduces its gem value sharply. More particular to our purpose here is that light can't be transmitted through the stone along ifs c-axis - a condition we require.
4. We need all our ducks in a row and along the c-axis. Light source, two (or more) slices of the same tourmaline cut at 90 deg to the c axis and stacked successively one on top of the next between the the light and the viewer. The thicknesses of the the slices , and their successive stacking all with the same c-axis orientation. should determine the usumbara effect colour change along the following lines:
- First slice < 1mm green, tending to colourless (colourless at 0mm).
- Add 2mm slice. Saturated green effect at 3mm.
- Add 2mm slice. Yellow-green
- Add 2mm-4mm slice Yellow.
- At 10mm. Deep orange.
- at 12mm. Deep red.
Those are the theoretical expectations
18th Jul 2013 16:32 UTCBarry Miller
-------------------------------------------------------
> Barry,
>
> It's hard to be sure without standing at your
> shoulder and watching but what you describe does
> not sound like usambara effect to me but more like
> dichroism. With pleochroism, the colour you see is
> determined by the crystal axis you view along.
Owen - Again, thank you for the helpful information. In follow-up to your reply about the possibility that my specimen demonstrates pleochroism and not the Usambara effect, I looked at it again with a flashlight in a dark room to see if I could see the red color while shining the light up through (i.e., parallel to) the "C" axis. I did, in fact, see the red color when I checked carefully. So, I am convinced that it does have the Usambara effect and not merely pleochroism.
Barry
18th Jul 2013 16:48 UTCOwen Lewis
So what two colours are you now seeing when viewing down the c-axis and how do you trigger a change from one to the other? Can you make a couple of pics?
18th Jul 2013 17:57 UTCElise Skalwold
For the best information on Usambara Effect, I strongly suggest reading the following three peer-reviewed papers which cover the original work by Asbjørn Halvorsen:
Halvorsen and Jensen, B.B. (1997) A new colour-change effect. Journal of Gemmology, Vol. 25, No. 5, pp. 325-330. (the first paper on Usambara effect. Letter from Dr. Kurt Nassau in following issue.)
Liu, Y. and Shigley, J.E. and Halvorsen. A. (1999) Colour hue change of a gem tourmaline from Umba Valley, Tanzania. Journal of Gemmology, Vol. 26, No. 6, pp. 386-396.
Halvorsen, A. (2006) The Usambara effect and its interaction with other color-change phenomenon. Journal of Gemmology, Vol. 30, No. 1/2, pp. 1-21.
If you do not have access to these, you may enjoy reading this essay which I wrote in 2008 in collaboration with Asbjørn and with the help of the late Dr. Kurt Nassau, entitled “The Usambara Effect: an overview of its nature” which can be read here: http://www.nordskip.com/usambara.html . At that time there was very little in the literature or on the Internet about UE. Asbjørn gave me the opportunity to study the original UE tourmalines in 2008-09 and later I received a gift from him of several of these very tourmalines. It is an optical phenomenon which I find entirely fascinating and a scientific experience which holds special significance for me.
Very best wishes,
Elise
18th Jul 2013 18:06 UTCBarry Miller
-------------------------------------------------------
> Barry,
>
> So what two colours are you now seeing when
> viewing down the c-axis and how do you trigger a
> change from one to the other? Can you make a
> couple of pics?
Owen,
The crystal in daylight is a rather dark green. From a distance, you would almost think it's black - but it's not. Adding to this is the complication - as you noted above - that light is not transmitted as readily through the "C" axis of tourmaline as it is transmitted perpendicular to it. I came across that characteristic while reading through the following gemological website: http://www.yourgemologist.com/tourmaline.html , which notes this fact when discussing orienting/faceting a tourmaline gem. So any hope of my rather dark-looking crystal providing readily seen/photographed color is not that great. Anyway, to answer your question, what I see is a narrow discontinuous outer band of red while looking down onto the "C" axis (i.e., looking down onto the termination) and I have to do so in a darkened room. I really don't feel hopeful about being able to photograph but I may try. The red that I see more easily is that which appears when I shine the light perpendicular to the crystal. If I'm able to photograph it successfully I'll post it but last time I tried I was unable to. I don't know if the Usambara tourmalines ever occur in a lighter, more transparent, green that would make photographing the effect much easier.
18th Jul 2013 18:19 UTCBarry Miller
-------------------------------------------------------
> Hi Barry,
>
> For the best information on Usambara Effect, I
> strongly suggest reading the following three
> peer-reviewed papers which cover the original work
> by Asbjørn Halvorsen:
>
> Halvorsen and Jensen, B.B. (1997) A new
> colour-change effect. Journal of Gemmology, Vol.
> 25, No. 5, pp. 325-330. (the first paper on
> Usambara effect. Letter from Dr. Kurt Nassau in
> following issue.)
>
> Liu, Y. and Shigley, J.E. and Halvorsen. A. (1999)
> Colour hue change of a gem tourmaline from Umba
> Valley, Tanzania. Journal of Gemmology, Vol. 26,
> No. 6, pp. 386-396.
>
> Halvorsen, A. (2006) The Usambara effect and its
> interaction with other color-change phenomenon.
> Journal of Gemmology, Vol. 30, No. 1/2, pp. 1-21.
>
> If you do not have access to these, you may enjoy
> reading this essay which I wrote in 2008 in
> collaboration with Asbjørn and with the help of
> the late Dr. Kurt Nassau, entitled “The
> Usambara Effect: an overview of its nature”
> which can be read here:
> http://www.nordskip.com/usambara.html . At that
> time there was very little in the literature or on
> the Internet about UE. Asbjørn gave me the
> opportunity to study the original UE tourmalines
> in 2008-09 and later I received a gift from him of
> several of these very tourmalines. It is an
> optical phenomenon which I find entirely
> fascinating and a scientific experience which
> holds special significance for me.
>
> Very best wishes,
> Elise
Elise,
Thank you very much. Your essay is excellent and the first discussion that I read on the subject.
Barry
18th Jul 2013 18:32 UTCOwen Lewis
> ....Anyway, to answer your question, what I see is a
> narrow discontinuous outer band of red while
> looking down onto the "C" axis (i.e., looking down
> onto the termination) and I have to do so in a
> darkened room. I really don't feel hopeful about
> being able to photograph but I may try. The red
> that I see more easily is that which appears when
> I shine the light perpendicular to the crystal.
> If I'm able to photograph it successfully I'll
> post it but last time I tried I was unable to. I
> don't know if the Usambara tourmalines ever occur
> in a lighter, more transparent, green that would
> make photographing the effect much easier.
Barry,
Do follow Elise's link and read her article. The pics should convince you that whether you are or are not seeing an Usambara effect. I still think, from what you write, that you are not.
18th Jul 2013 18:41 UTCOwen Lewis
Thank you very much. You mention in your article that the usambara effect has now been detected in a wider range of gemstones. Would you be able to give a source for any published papers on these additional discoveries? And in particular, any work you know to have been done since you wrote in 2008? I would be very grateful for this information.
Kind Regards,
Owen Lewis
18th Jul 2013 20:24 UTCBarry Miller
-------------------------------------------------------
> Mike, I don't like to pour cold water but this
> does not look right to me, for the following
> reasons:
>
> 1. You at showing a strong 'tourmaline' pink. This
> is not one of the colours predicted for usumbara
> effect and is commonly produced where Mn trace is
> the chromophore. Bi-coloured/multicoloured stones
> are not suited to proving usambara effect. I'm not
> saying it can't be dome only that it has not been
> done and you are making life unnecessarily
> difficult for your self by trying.
Owen,
Actually, it appears to me that the crystal that Mike illustrates is an example of the Usambara effect. It does not look like a bi-colored crystal as I can see little areas of green at the termination which turn the reddish color upon shining the light through it. If the crystal were bi-colored with green occurring at the termination, I don't believe the reddish color would appear as it does since the green would obscure it. Regarding my own crystal, I still believe it to be Usambara as well. Just a friendly disagreement.
>
>
18th Jul 2013 21:02 UTCOwen Lewis
So, it's all up to you.
19th Jul 2013 01:09 UTCMike Keim
No worries about "pouring cold water", but I do want to figure out what is going on because I believe what I'm seeing is the Usambara effect. I have read Elise's essay years ago when I originally got my crystals, and what I see seems exactly what is described in her essay. First let me address your comments:
1. You at showing a strong 'tourmaline' pink. This is not one of the colours predicted for usumbara effect and is commonly produced where Mn trace is the chromophore. Bi-coloured/multicoloured stones are not suited to proving usambara effect.
To my eyes, here in hand, the color change is from a green to a red, maybe purple/red, not a pink. Not sure why you see a pink, maybe due to my photography or computor monitor variability. The colors I see are the same as show in Elise's essay. You also seem to think the crystals I am showing you are bi-colored in regular light - they are not, they are a uniform dark green until you shine a light up the C-axis.
2. You are rotating the stone noticably between the two shots in both the horizontal and the vertical plane. This means that at least one of the two shots is not taken along the c-axis and that some pleochroic colour alteration must be be occurring.
They may be slightly off plane and rotated in the photos, but it has no effect on the color, if the two photos were perfectly aligned they would look the same. I can rotate them in hand or tilt them back and forth and it makes no difference in the red color.
3. The largely black appearance of the specimen in the first pic suggest that this specimen exhibits a 'closed' c-axis. An unhelpul and common condition in gem tourmaline that reduces its gem value sharply. More particular to our purpose here is that light can't be transmitted through the stone along ifs c-axis - a condition we require.
I'm not sure what you mean by "closed c-axis"? If you mean it is opaque, it is not, just dark green - clearly the bottom photo shows that light readily passes up the c-axis. I'm not sure if you have ever handled any of these Nchongo crystals, but all that I have had are relatively flat, button shaped crystals with the width of the termination face being much wider than the depth of the crystal. In both photos you are looking directly down on the termination (down the c-axis)
4. We need all our ducks in a row and along the c-axis. Light source, two (or more) slices of the same tourmaline cut at 90 deg to the c axis and stacked successively one on top of the next between the the light and the viewer. The thicknesses of the the slices , and their successive stacking all with the same c-axis orientation. should determine the usumbara effect colour change
I agree that having slices of the tourmaline are one way of showing the effect (i.e. if the critical thickness is 6mm, taking two slices of thickness 3mm and showing the color change to red when you overlap them), but I do not think it necessary. Why is not a natural crystal with areas of thickness that vary between less than and greater than 6mm (hypothetically) not suitable to show the effect? In fact Elise's essay shows a crystal that shows the Usambara Effect.
You also wrote:
"In conclusion the usambara effect is rare. Despite well over a century of intensive study of the interplay of light and gemstones, it was not identifiec and documented until 1997. You need gem grade tourmaline with an appropriate chromophore. You then need to prepare a small range of slices that you can stack up on each other. Testing for usumbara effect is painstaking. The odds against spotting it 'in the wild' - or even in a faceted gemstone are long."
I believe, and as shown in Elise's essay, you can see the effect in a whole crystal. I agree it is a rare effect worldwide, but of the 20 or so Nchongo crystals I had, all of them showed this color change rather easily. I have attached two more photos that might help show what I describe. The first show the crystal somewhat from the side and you can see that the right side is much thinner. The left side of the crystal is 1.1cm thick and grades down to 2mm thick on the right side. It is 2.5 cm wide. The second photo is looking directly down the c-axis, and you can see that the thicker left side of the crystal changes color while the thinner right side remains green.
The effect was first described on crystals from Nchongo, Umba Valley, Tanzania, my crystals are from this locality, I'm seeing a color change up the c-axis just as shown and described in Elise's essay, so I believe the Usambara Effect is what I'm seeing in my crystals.
19th Jul 2013 10:22 UTCOwen Lewis
OK, I'm intrigued but, like Doubting Thomas, I have to push my fingers into the wound in the side of the living Christ before I believe.
I'd like to chat ways and means for me to get the specimen you have last shown, just above, on my bench. PM follows in about an hour from now. I really hope we can work something out.
Bottom line is that, yes, of course is is possible to show an usambara effect in a whole stone - the difficulty lies in showing beyond doubt that an observed colour change results from a change in compound thickness only and no other effect. Easy with slices. Harder with a single piece. More in a while. gotta go now.
Best regards,
Owen
19th Jul 2013 14:45 UTCElise Skalwold
Best wishes,
Elise
PS nice UE photos Mike! I remember when you first posted these great specimens to your website.
19th Jul 2013 15:51 UTCAlexander Ringel
Thank you very much. You mention in your article that the usambara effect has now been detected in a wider range of gemstones. Would you be able to give a source for any published papers on these additional discoveries? And in particular, any work you know to have been done since you wrote in 2008? I would be very grateful for this information.
I got a sample of brazllian alexandrite (typical dark twin), which is in thin layers in daylight green and in thick layers in same light red. This sample got of course also a perfect color change.
I got also a dark red painite. Inner reflections short under the surface are greyish green without any red hue. Inner reflections of deeper areas are brilliant red. This sample got also a perfect color change. All greyish green becomes reddish under incandescent light.
The thickness of the required layer can be varied with changing of the light source. A bluish white LED light source increases on both samples the thickness, which is required for a color change. This is prove enough for a proper usambara effect.
OK, I'm intrigued but, like Doubting Thomas, I have to push my fingers into the wound in the side of the living Christ before I believe.
I'd like to chat ways and means for me to get the specimen you have last shown, just above, on my bench. PM follows in about an hour from now. I really hope we can work something out.
Its with no doubt a usambara effect. Stones like this are nowadays offered in many online stores for a few bucks. So there is no reason to ship such a stone around. I got of course also such a sample. And same thing like with above mentioned samples. Thin layers green, thick parts red. LED light needs thicker layers to become red.
19th Jul 2013 16:32 UTCOwen Lewis
Many thanks for the information re. Uni Nantes. I have some reading to arrange....
And you are right about Mike's pics. I've just arranged a purchase from him. Only sorry that I found my way to his site so late.
Kind regards,
Owen
19th Jul 2013 16:58 UTCOwen Lewis
-------------------------------------------------------
> I got a sample of brazllian alexandrite (typical
> dark twin), which is in thin layers in daylight
> green and in thick layers in same light red. This
> sample got of course also a perfect color change.
>
>
> I got also a dark red painite. Inner reflections
> short under the surface are greyish green without
> any red hue. Inner reflections of deeper areas are
> brilliant red. This sample got also a perfect
> color change. All greyish green becomes reddish
> under incandescent light.
>
> The thickness of the required layer can be varied
> with changing of the light source. A bluish white
> LED light source increases on both samples the
> thickness, which is required for a color change.
> This is prove enough for a proper usambara effect.
Now that, Alex, is very interesting too, If you ever do a write up on your observations with these stone, including full detail of light flux strength and spectrum analysis of the various light inputs against those of the light outputs, I'd be very interested to read - and I'm sure plenty of others would also.
19th Jul 2013 19:56 UTCAlexander Ringel
I noticed this effect during my observations.
In the example of my usambara tourmaline, the red area becomes bigger under incandescent light and becomes way smaller under bluish LED light.
In the case of the painite, the area of greenish inner reflections is larger under blue LED light, than under daylight.
On alexandrite under LED light is still some green on the backside of the crystal visible. Sunlight lets the backside look only red.
19th Jul 2013 20:53 UTCOwen Lewis
Alexander Ringel Wrote:
-------------------------------------------------------
> Im not a scientist, just a collector. So you can
> threat this as write up.
>
> I noticed this effect during my observations.
>
> In the example of my usambara tourmaline, the red
> area becomes bigger under incandescent light and
> becomes way smaller under bluish LED light.
There is a reduced red component in the light from many white LED. LEDs differ and I can't say what yours does. But it you have its make and type number, you can find out.
Incandescent light also varies, mainly according to how hot the filament is run. But all tend to be blue-deficient with a heavy red and IR output. This means that *in general* if you look at something illuminated with an LED and it shows some red, expect to see more/stronger red when its illuminated with an incandescent source of the same overall output.
This is what pesky 'scientists'need to do when making observations from which to draw conclusions to share with others. It is not what the rest of the world always does. The rest of the world, especially those selling alexandrite will show the colour change in a stone between viewing it in midday sunlight (blue-heavy) and then by candle-light (no blue and all yellow/orange/red/IR). This shows the best (strongest) alexandrite effect for that stone (other than in special lab conditions)
I think that to show usumbara effect in an alexandrite, you might first have to show observed colour change as occurring only when stone thickness in the transmission path is altered and where there is *no* change in the quality or strength of the illumination. With anything else - especially with a green-red change, how and you be sure that alexandrite effect is not causing part or even all of the colour change you see?
>
> In the case of the painite, the area of greenish
> inner reflections is larger under blue LED light,
> than under daylight.
More blue in the illumination?
> On alexandrite under LED light is still some green
> on the backside of the crystal visible. Sunlight
> lets the backside look only red.
Mmmm..... strong sunlight is blue-heavy as is (probably) your LED, so how much stronger/weaker than sunlight is your LED output at the point they enter the stone?
If it was me, I'd want to work with a single light source (of known characteristics, if one is going to be pernickety)
Not to worry. It's been thought-provoking having your observations and helpful to me is finding my own way ahead.
Thanks.
Owen
20th Jul 2013 00:05 UTCAlexander Ringel
http://www.ebay.de/itm/5-x-GU10-LED-Lampe-Ersatz-fur-Energiesparlampe-Halogen-/230627696122?pt=DE_M%C3%B6bel_Wohnen_Leuchtmittel&var=&hash=item35b27b5dfa
(Type: "weiß")
I use them for checking certain colors of stones, because they are more similar to daylight, than incandescent light. But they do also modify the colors. The light of these LEDs is "cold", which means, they are more bluish than daylight. I have an self constructed CD spectrometer, on which i can see a more or less continously spectrum with a small gap at hte border between red and green and another at hte border between blue and green. Still way better than the spectrum of energy saving lamps (which constists of multiple fine lines).
If it was me, I'd want to work with a single light source (of known characteristics, if one is going to be pernickety)
I use mostly daylight, but winters are very dark here in germany, so i have to use artificial light. Incandescent light is good for most stones, but for color changing (alexandrite,...) and color varying (sugilite,...) stones i need often another source of light, which influences the color not so much.
With anything else - especially with a green-red change, how and you be sure that alexandrite effect is not causing part or even all of the colour change you see?
The red is also visible in sunlight. The thin rim of the crystal is green, the thick inner part is pure and perfect red. Under LED the alexandrite is way more green. Only some red patches are visible on the thick inner part. Under incandescent light, the alexandrite is completely red (except of a bluish red color in thinnest parts). So i conclude, that color change is not the only effect, which affect the color of this alexandrite. My other 2 alexandrites dont show any usambara effect at all. They show only a (almost) perfect color change from red to green.
Same with my painites. Only one of my 5 samples shows a usambara effect. The remaining 4 show only slight till medium color changes.
20th Jan 2015 02:53 UTCAnonymous User
I recalled seeing the UE tourmalines on Mike's site a few years ago, and have regretted not getting any.
Now I've got one! It fits the general appearance of the ones that Mike has posted, and matches the details of the one described by Barry (the OP) very closely. The red color is only easily apparent when the tungsten bulb light (or sunlight) is directed perpendicular to the C axis. Interestingly, the red color is sharply defined as only half the crystal, with the dividing line apparent from the side as a division perpendicular to the C. (This division between red and green is not due to differences in thickness.) Parallel to the C with the same light source the color is nearly all green. With an LED light, the color is all green both perpendicular and parallel to the C.
I agree with Barry that taking photos showing any of the colors is difficult, due to the stone's strong saturation.
I have only the one stone, and cannot slice it. So I looked for other ways to confirm the presence of the UE.
The stone shows up as all deep red in the Chelsea filter, suggesting chromium.
If an ordinary green "chrome" tourmaline (also from Tanzania but with no evidence of UE or alex effect) is held beneath the subject stone, and the tungsten light is shown up through both along the C axis, where they overlap deep red shows.
When the subject stone is placed on top of green leaves (not thick enough to show as red on their own), and light is directed through both, the stone shows as red. This is demonstrated in the recommended article.
When the stone is placed on top of a mirror, and light is directed into the stone and reflected back out through the stone (effectively doubling the stone's thickness), deep red shows.
What I conclude I have is a UE tourmaline that shows a strong effect for half of the length of the prism, which makes the red visible in that half of the stone (the non-terminated end) when viewed perpendicular to the C axis. The other end apparently has less of the required chromophore. Since when viewed down the C the light is passing through 1/2 thickness of strong UE and 1/2 of weak UE material, effectively no red shows through. But when helped (with another tourmaline, leaf, or mirror) the red shows strongly through the C.
Of course in general there is pleochroism. The shades of green shown perpendicular and parallel to the C are different. Because pleochroic colors and the UE colors are different in different light sources, there may be some alex effect color shift or color change. So the stone may show all three color effects.
If I can take decent pictures I'll post them.
Thanks as always for great information!
21st Jan 2015 01:43 UTCAnonymous User
For the photo taken on the mirror, please note that the light going through twice is really deep red instead of the purple that the picture shows.
Thanks.
27th Jan 2015 21:45 UTCBarry Miller
28th Jan 2015 01:53 UTCAdam Kelly
Guess I'll have to try my hand at photographing the pieces I have.
AK
28th Jan 2015 17:40 UTCAnonymous User
Adam, please post yours.
Owen, did you verify that the stone you got from Mike shows the UE?
26th Jan 2016 21:29 UTCDan Costian
http://www.mindat.org/photo-728952.html
http://www.mindat.org/photo-728953.html
27th Jan 2016 11:17 UTCOwen Melfyn Lewis
-------------------------------------------------------
> Thanks for the photos, Barry. It is very hard to
> take representative color photos of these very
> deeply saturated stones.
> Adam, please post yours.
> Owen, did you verify that the stone you got from
> Mike shows the UE?
Ken,
Don't know how your post escaped me for very nearly a year but it did (until Dan bumped the thread just now). Sorry to have teken so long to respond.
Yes, UE confirmed. Not the best example as the crystal is quite dark. I want to have it sliced and polished to produce a nice demonstration set but won't unless and until I can acquire one or two more specimens. I keep my eyes open but they are really not easy to come by.
27th Jan 2016 16:58 UTCTony Albini
Thanks to all for a discussion on a rare phenomenon in minerals. You learn something new every day. I think that is why we enjoy this hobby.
30th Jan 2016 22:19 UTCAnonymous User
I've since had my UE tourmaline piece faceted by Jerry Newman. (For pictures of the rough before cutting, and my long-winded descriptions of it, see above in the thread.) The faceted stone is really dark chrome green with transmitted light, and deep red with reflected tungsten light. I was hoping to thus capture the UE in the cut stone, and it worked!. The stone is too saturated to be considered to be attractive to most, but I love it because of the effect and success story.
Thanks.
Ken
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