Post subject: Making a "color change" gemstone with the Usambara effect.
Posted: Sat Sep 16, 2017 4:20 pm
Gemology Online Veteran
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
A few months ago I found an interesting advertisement on the internet. A British firm had a color change tourmaline that went from red to green. It was quite dark and the color change was only seen when you looked down on the gemstone face up.
Now I had imagined such a gemstone, but I did not really expect to see one of them for sale. I had to spend the bucks to get a real life view of this "very rare" tourmaline. I wanted to get my spectrometer together before I reported on my find, so I delayed writing about it even after I received the very dark, less than a carat oval.
My spectrometer had a simple adjustment made on it by the company and I am rolling along, so I tested the "color change" tourmaline. It is a chrome rich dravite, I have a similar example in my collection and it is so dark that the best way to see it is with the light for my spectrometer.
When you look down on the oval with a bright light behind you, the thicker parts of the gem appear a bright red, If you tilt the stone, you can see green edges in the gem and you get the same green if you look at the gem from the pavilion side. This play of color is pure usambara as the tittle of this post suggest. It is not "color change" as defined for gemstones because it depends of the length the light travels threw the gemstone and not on a change in the make up of the light source. Face up the light travels far enough for red color to dominate while off axis or from the pavilion the path of the light is too short for the red color to dominate and green dominates.
To make such a gem you would have to start out with a piece of usambara tourmaline and grind it thin enough that you can no longer see red when you look threw the piece. Now dop the piece using one of the flats you have ground on the piece as a table. If you don't cut the gemstone too thin by staying reasonably close to the thickness that you had when you could not see red threw the piece, you should have made a nice "color change" red to green tourmaline,
If there is anyone out there with a true red to green, color changing tourmaline I would greatly appreciate seeing pictures of it. It is impossible to photograph the color effect in the stone I have described with my equipment. I have a Dravite the changes from a desaturated yellow (yellowish straw color) to a green, but I don't believe that a true red to green color changer exists. (I have read that it exists in both books and on the internet.) Please prove me wrong.
Post subject: Re: Making a "color change" gemstone with the Usambara effec
Posted: Sat Sep 16, 2017 9:51 pm
Valued Contributor
Joined: Fri Dec 25, 2015 8:31 pm Posts: 276
Bruce, would you happen to have any green tourmalines that have a clear absence of yellow in their spectra? I have heard that chrome tourmalines have an absence of orange in their spectra, but the only colour change I have been able to confirm in regular material is from salmon to lavender(■>■). This occurred in the C axis of a long pink crystal with a brown rind when switching from halogen lighting to sunlight.
I imagine reddening the yellow part of the shift in chrome could be possible though the Usambara effect, so you get a yellowish-green to orange-red effect occurring, but it would be anything but a consistent and attractive colour change.
Post subject: Re: Making a "color change" gemstone with the Usambara effec
Posted: Sun Sep 17, 2017 4:50 pm
Gemology Online Veteran
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Thanks for the replies, Marlow I did talk to the suppliers you mentioned and they purchased a large amount of the material sometime ago. I tried to get a piece of rough, but was unsuccessful. I would love to see the two stones in person, but I am sure from the description that they are usambara effect chrome colored dravite like I described.
Pure yellow is an interesting color in tourmaline. It's absorption graph shows a peak in the edge of the violet end of the visible spectrum and then no peaks at all in the visible range of frequencies. If there are other peaks it tends to give an overtone to the yellow. At times orange looks a lot like yellow, but its peak is closer to 500nm and there are no peaks in any redder frequencies. There can be absorption in the bluer end, below 500nm and still have an orange, but how pure it is would be difficult to say. Now for green. With iron the common chromophore/coloring agent, you have a broad peak in the 700nm red range and usually a peak around the 400nm range (from manganese Mn+2). If the graph between those two peaks slopes down toward blue you will probably have a blue stone and a slope toward the red end usually produces a green to yellow green in ELBAITE. With Dravite the world changes and there are two peaks of roughly equally saturation in Chrome tourmaline. One is centered on the little redder side of 600nm which is in the orange range of the visible spectrum and the other is center roughly on 500nm in the green to blue range. It is quite distinctive and easy for the spectrometer to tell Chrome fDravite from green Elbaite because I have never seen a 600 nm peak with Elbaite of any color. Finally I find that the color of the Chrome tourmaline looses it high saturation as the 600nm peak is reduced relative to the 500 nm peak. As the imbalance grows stronger, I begin to see color change such as the one I described going from green to wheat. I have talked with a person that has a color changer that goes from green to orange, but as I have said, never one that went to red without the usambara effect.
I hope this is some help, though I doubt I have answered you question. I would be happy to test any cut stone with my spectrometer for the wonderful effect of color change.
Post subject: Re: Making a "color change" gemstone with the Usambara effec
Posted: Thu Sep 21, 2017 3:47 am
Valued Contributor
Joined: Fri Dec 25, 2015 8:31 pm Posts: 276
bruce_tourm wrote:
I hope this is some help, though I doubt I have answered you question. I would be happy to test any cut stone with my spectrometer for the wonderful effect of color change.
I blame my inexperience with your setup here - The only optical spectrometers I've used are the cheap hand ones you can get for under $100 Your answer does give me the information I need though.
In your case the lack of yellow should translate to a peak around 560 to 585nm, from what you have said about both green, yellow, and blue tourmaline colours definitely points to it not being possible in elbaite. That 600nm peak in chrome tourmaline makes me think you could get a more orange colour change if you used a redder light source for the change at least, which could explain that person's example. Below 2700K might work but I don't expect lights of that colour to become popular anytime soon. It's a worse position than those garnets that were discovered to change to purple under cool lights...
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