This dark flash driven green emerald cut is a Dravite, the species, and displays the usambala effect. It appears to be eye clean and weighs 1.12 carats.
I will be posting more on the usambara effect in tourmaline, but I will focus more on this specific gemstone in this post.
It is always a pleasure to discover something new about tourmaline and then to discover something unexpected in one of your gemstones is even better. While I was testing my collection for copper with my spectrometer, I saw red flashes from the intense light used by the spectrometer, in a green emerald cut. Now I had never seen that before, but I had an idea what it was. Usambara is an unusual property of some substances to absorb different wavelengths of light to different degrees. In the usambara effect, green is absorb more strongly than red.
This gemstone is not big enough to see the change from green to red that occurs with the usambala effect, but I have confirmed its presence. The effect only occurs in Dravite, the species, and needs to have chrome as the chromophore. This emerald cut appears to be eye clean and with decent crystal. It has a dark flash drive color which is good for usambala tourmaline which many times it too dark to make effective gemstones. It weighs 1.12 carats.
Bruce
