Usambara effect, chrome chromophore, green emerald cut.#420

Dark flash driven green emerald cut, usambala. 20 #4 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.


About Bruce Fry

I was born in Summit, NJ in 1947 and graduated from Summit High School in 1966. I graduated from the Colorado School of Mines in 1970 and after spending another year in graduate school, I left to see the world of Brazil. After spending some more time discovering myself, I ended up working for the Commonwealth of Pennsylvania for 32 years as an Air Quality Engineer in the Department of Environmental Protection. I retired in 2007 and took up faceting gemstones again after a long hiatus that reached back to my twenties. I had started cutting cabochons when I was 13 and bought my first faceting machine when I was 15, but ran out of money and time until I retired. My great love in gemology is tourmaline and the collection presented here represents my effort to get as much beauty and variety in the colors of tourmaline as I can. I was particularly lucky in being able to get unheated cuprian tourmaline before copper was discovered in gem grade tourmaline from Mozambique.
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