This tourmaline has 100% Color change this does not behave like a Usambara or Dravite tourmaline. This stone changes Color in a single axis like an AlexandriteThough this appears dark without light, this is a phenomenal stone with a phenomenal size 2750 Carats. Explain this phenomenon.
Discussion here on the IGS forum of the Usambara effect notes that it may be a combination of path-length dependent absorption and the alexandrite effect, so it’s hard to say what exactly is happening with your stone without seeing it in 3D and trying different light sources and light paths. It would be interesting to know what light sources you are using in the picture.
Thanks for your reply. While green - LED White light and while Red it is mere candle light. The same Change occur even with incandescent light.
It is 4750 carats.
Awesome!! Thank you
The color change is due to the balance between color wavelength absorption rate vs. wavelength quantity.
Since the LED emits all wavelengths of light at a greater intensity than the candle light or incandescent light, more of that light, in all wavelengths, comes through. Focus towards the bottom of the sample, and you will see that it will appear more yellow there, indicating more of the green has been absorbed.
The candle/incandescent light source does not emit at the same over-all intensity, and does not emit green as strongly as yellow and red, you get the color change phenomenon. Pay close attention to the top-right corner of the sample, and you will see that the emitted color is an orange-yellow, indicating that less of the green is being absorbed.
In short, the intensity of the LED source is over-powering the usambara effect.
Even with a very low intensity white light with a pen torch the stone emits green color.
Usambara effect does get affected by pleochroism as present in tourmaline.
Is this stone for sale?
Hello, Yes this stone is for sale. Quote your reasonable offer. This is in fact 4750 carats and it was an inadvertent mistake to mention it as 2750 carats. This has a tourmaline certificate from GIA. This stone will look dark when viewed in normal light. Even with a faint light it lights up. The stone is transparent.