What is the Usambara Effect In Gemstones?

Question

I'm an engineering geologist mainly involved in tunnel construction. I also collect gemstones. My special interest is in color change minerals. I've been involved in studies of a special color change phenomenon, the Usambara effect. I'm interested in hearing if any members of the International Gem Society (IGS) know this effect and in what minerals they've seen it.

Articles on the Usambara Effect in Tourmalines

A little background. My work has brought me frequently to East Africa. In the Umba area of Tanzania, I got hold of some chrome tourmalines that showed a color change different from anything I'd seen described before. Together with Brenda Jensen at the Geological Museum in Oslo, I wrote an article about this phenomenon for the British magazine, The Journal of Gemmology. The introduction to this article was as follows:

The Journal of Gemmology, 1997, Volume 25, No. 5. "A New Colour-Change Effect.." A. Halvorsen and B.B. Jensen. pp.325-330. Crystals of green chrome tourmaline from the Umba Valley (Tanzania) may show a wine-red colour when viewed in certain directions. This is neither pleochroism, nor a colour-change of alexandrite type. The change from green to red depends primarily on the path length of light through the stone and probably also on the content of Cr and V; in the samples tested the stones appear red when more than 15 mm thick. It is suggested that this new type of colour change be called the Usambara effect.

In 1998-1999, I also participated in another article together with Yan Liu and James Shigley of the Gemological Institute of America (GIA). The introduction was as follows:

The Journal of Gemmology, 1999, Volume 26, No. 2, "Colour Hue Change of a Gem Tourmaline from the Umba Valley, Tanzania." Y. Liu, J.E. Shigley, and A. Halvorsen. pp 386-396. The hue of a tourmaline from the Umba Valley, Tanzania, changes from green to red with increases in path length of light through the material. This hue change phenomenon, called the "Usambara effect," is different from any colour-change effect previously described in gemstones. Colorimetric methods have been used to analyze this tourmaline. Results of this study suggests that the Usambara effect is a complex phenomenon including effects of both path length and the colour change observed under different light sources (the alexandrite effect).

More Gemstones with the Usambara Effect

I've since found the same color change behavior in other minerals and from other localities. I've observed the effect in alexandrite, epidote, garnet, sapphire, scapolite, purple spinel, and tanzanite. All minerals showing the alexandrite effect may also have a component of the Usambara effect. If a reflected light ray has a color different from the body color of the stone, one should be suspicious. In some cases, this might be a result of dispersion, but it might also be caused by the Usambara effect.

I would like to hear about observations of the same, or similar, color behavior in other minerals.

Sincerely,

Asbjørn Halvorsen, Son, Norway

Three Usambara Effect Tourmalines

I've cut three pieces of tourmaline with the Usambara effect and found them quite unusual. One piece of rough showed the red/green effect. I tried cutting it "between" the two colors. As a result, I got a very dark stone that didn't give the effect I was hoping for. (I thought it might come out similar to andalusite). Another piece showed a golden color on one axis and brown on another. I cut a piece of this for the golden color and got a decent looking gem. I cut the third piece for the brown in a "Gram Princess." That stone came out really nice, dark with golden flashes.

Norm Holbert, Norwich, CT

Advice For Cutting Usambara Effect Gemstones?

I recently acquired some of this material and was quite fascinated with the Usambara effect. I'd never seen it before. Can anyone give me some suggestions on how to cut this stuff to show the effect? It appears that the crystals could easily be oriented to show either the chrome green (c axis) or the red (a-b axis) color. Is there a cut or orientation that could be used to show both?

Thanks,

Richard Winnick

An "Emerald Filter" Effect?

My chrome tourmaline stone, cut from a rather dark crystal tip, will flash VERY red highlights at times. I don't recall its country of origin. Perhaps it's behaving like its own "emerald filter" and showing the other narrow spectrum portion? Odd.

Carl 1 Lucky Texan

A Follow-Up on Cutting Usambara Effect Gems

It is a challenge getting a good result when cutting Usambara effect stones. I'm not a cutter myself. In general I consider them collectors' stones more than jewelry stones.

I have had several stones faceted by various cutters with varied results. First of all, a good result depends on precise cutting. Most stones with good color change seem to be on the dark side. Stones that are too large will not yield good brilliance. Good brilliance in general is desirable. Small stones (cut size less than 1-2 carats) may be best as brilliants. I have stones as small as 0.15 carats with red flashes. With successful cutting, however, the result may be something really special: a firework of red flashes in the otherwise deep green stone, due to the different path lengths of the light rays. When I had my first Usambara effect stone cut, I was wondering if it would be a dirty mix of green and red. Instead, the result was a dramatic, pure, separated green and red.

In large stones, step cuts with large facets may be interesting. Pleochroism may affect the result. If the stone has pronounced pleochroism, orient the stone correctly. However, in most stones I've seen, pleochroism is of minor importance. In such cases, the shape of the rough should direct the orientation.

Direct sunlight seems to be the best illumination for studying a cut stone. It gives both bright green and red reflections. Incandescent light gives more of the red. With fluorescent light, you won't see any red at all, only green.

A Follow-Up on the "Emerald Filter" Effect

The tourmaline spectrum is indeed similar to an "emerald filter" spectrum. If you stack several emerald filters on top of each other, you'll probably find that the color of a transmitted light ray changes to red. I've found green plastic bottles with the same effect. (In the United States, I've also found blue soda bottles showing color change to red).

Additional Comments on the Usambara Effect

The Usambara effect is a complex color change phenomenon related to a certain pattern of the transmittance spectrum similar to the alexandrite pattern. A color change may occur when the balance between the green and red transmission bands shift as a result of changes either in the optical absorption in the stone or in the spectral composition of the illumination. Conditions influencing the optical absorption are: path length of transmitted or reflected light rays, chromophore concentration, and the orientation of light in relation to the crystallographic axes (pleochroism).

For a particular stone, the chromophore concentration is fixed and the change in optical absorption is influenced by the change in path length and by the orientation of light in relation to the crystallographic axes. With a general increase in absorption (for instance, by increased path length), the absorption in the higher frequencies (the violet part of the spectrum) will increase more than in the lower frequencies (the red part of the spectrum). As a result, the balance between the green and red transmissions shifts towards red, due to an increase in the intensity of the red transmission relative to the intensity of the green transmission. When this shift in balance reaches a certain point, the human eye will suddenly see only red, not green.

When Was The Usambara Effect First Observed?

Locals in the Umba area in Tanzania suggest that digging for chrome tourmalines was initiated in the mid-1960s by Papas, a legendary Greek. Even if people have been digging for these stones since then, it seems few are aware of the color change effect. My assumption is that most of the tourmaline material has gone to Asia.

When I sent some samples to GIA in 1995, James E. Shigley, Director of Research, replied in a fax: "We have not yet been able to explain the color appearance of this material but we have some ideas." A study of probably the same color change behavior in garnets was presented by D.V. Manson & C.M. Stockton of GIA in Gems and Gemology, Winter 1984. In addition to the alexandrite effect of the garnets, they observed that the colors of reflected light rays were different from the body color of the stones. Regarding this latter effect, they concluded: "As yet, we have not been able to determine the optical or physical mechanism whereby this phenomenon occurs."

As far as I know, the Usambara effect was first described in the 1997 article in The Journal of Gemmology.  In January, 2000, the French geologist Cedrik Simonet described the Usambara effect in tourmalines from Kenya in that same journal. He later reported to me personally that he had seen the same effect in garnets from Kenya.

Best regards,

Asbjørn

Further Reading

Our entry on the Usambara effect in the phenomenal gems article also discusses this effect.

For more photos of Usambara effect gemstones, please see Elise Skalwold's article.