Chrome-tourmaline displaying the Usambara effect. The parent photo is the reference without the Usambara effect, therefore, to see the color change you should navigate through the children photos. From Merelani Hills, Tanzania. Enjoy!
That's why I am posting a link and not a picture so that everybody can find children photos there navigating with the arrows at the right (or left) of the photo in the link.
Post subject: Re: Chrome tourmaline, Usambara effect
Posted: Sat Jul 09, 2016 3:34 pm
Active Member
Joined: Fri Jun 08, 2012 11:33 am Posts: 75
Thank you, Marlow. I noticed that my link to the child photo doesn't work, therefore, for those who failed to open it I add here its picture (which is the most astounding of all).
Post subject: Re: Chrome tourmaline, Usambara effect
Posted: Thu Aug 11, 2016 9:27 pm
Gemology Online Veteran
Joined: Thu Sep 25, 2008 10:33 am Posts: 840 Location: Mars PA
Unsambara effect is very interesting and I am not surprised that it is displayed by the most interesting of gemstones, Tourmaline. It is displayed by substances that absorb different wavelengths of visible light at different rates. Therefore if you had equal levels of say red and green in your light source at the start, they would not be equal after passing threw a substance demonstrating the unsambara effect. The tourmaline is demonstrating the effect all the time. but only in pieces that are thick enough, can the eye pick up the spectral change that results in the gemstone transmitting green to red light. You can see in the picture that there are areas of yellowish color that comes from having the right thickness to mix both red and green together to make yellow.
My first moment with the effect was when I put my spectrometer's light to the table of a small emerald cut with a spinach green color. I had not bought the gem as a unsambara tourmaline, but the red flashes that came out of the green tourmaline told me something was different. I love differences. Later I was able to find a collector with a similar tourmaline. Both were too small to let the eye see the color change (This is not a color change in the way it is generally accepted in the trade.), but when we placed them table to table and transmitted light threw them, they became red in their thicker parts and remained green at their edges.
A classic experiment with the effect is done with chlorophyll in water. You demonstrate the effect by pouring the liquid into a glass and when it gets to be deep enough, the color of transmitted light changes. I don't know of any other gemstone that demonstrates the effect nor do I expect a great demand for tourmaline that displays the effect, since most examples I have seen are dark and not that attractive in everyday light. You have to set out to appreciate the effect in my opinion, just like real color change takes some effort to see.
Post subject: Re: Chrome tourmaline, Usambara effect
Posted: Fri Aug 12, 2016 12:39 am
Gemology Online Veteran
Joined: Mon Jun 16, 2014 1:10 pm Posts: 850
Thank you, Stephen for the picture and Bruce for ( as usual ) very interesting informations.
A german gem and mineral desler in Germany ( Mawingu ) offers thin slices of extremely dark green tourmaline - with backlight and two slices you see it too.
But Danco item is much lighter and better.
I have another gem with an usambara effect - a chrysoberyl with a very high chrome content - so purplish red in daylight.
On a cloudy day if you look through the pavillion the thinner edges and half of the stone ( a twin ) are bluishgreen. If you turn it the stone is dark purplish red.
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