Usambara effect color change Demantoid Garnet


Recently I’ve been lucky to mine some Demantoid garnets with the Usambara effect which seems to be rare and cool.

Do you know any Gemology lab that certifies the Usambara effect?
Is there any authority with extensive experience with Usambara stones?

also please see the pictures for your reference (stones don’t change color under incandescent light or white light)

Thanks for your consideration

Light enters the facet from the girdle and the stone’s color changes after the required thickness

The same rough stone viewed from two different thicknesses

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You have a bi-color demantoid, not the Usambara effect. The attached photo is an example of the Usambara effect in a chrome


Dear Jeffery,

thanks for your valuable reply.

for sure I’m completely familiar with bi-color or regular color-change gemstones especially garnets, as I do both mining and cutting.

also, thanks for the Tourmaline picture, the problem with my material is that the majority of Demantoids are so small, although this facet is 0.9 ct and is quite big for a Demantoid with such a phenomenon, but I can not view this effect in a 6mm facet as it’s shown in the mentioned tourmaline.

the following pictures are from the same facet but in non-biased lighting; the facet is completely unicolor with no color zoning or bi-color effect.
Also, you may consider the pictures of rough which are viewed with different thicknesses, and it’s unicolor.

Just like you, I couldn’t believe my eyes for weeks, but after hours of experimenting and viewing the rough and cut stones I’m almost sure about the Usambara effect

What’s your idea?
Any lab to certify the Usambara effect?

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Light path length is inducing its “dichromatism”

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Thanks a lot for your precious reply,

Dichromatism is called Usambara effect in gemstones right?

Do you know any gem lab that certifies such stones?

It depends on what your definition of is, is.
Dichromatism is not another word for Usambara.
I am not convinced that your stone is exhibiting the Usambara effect.
You would need to contact gem labs to see if they would certify this.


Another possibility is that If you do have a garnet (which is not a pleochroic stone) that is not bicolored, and are still seeing different “colors” - it could also be the concentration effect (due to impurities that absorb wavelengths) rather than the usambara effect. Based on the photos (faceted and rough), it does not appear that you are seeing different hues, but moreso different values due to absorption (based on the direction and intensity of the light passing through the stone vs internally reflected light).

See this article:


Thank you so much for your valuable concideration, yeah for sure need to be examined by a professional in color changing phenomena.


Thanks a lot Taurean,

The article you attached is awesome I started reading it a week ago, as you say it maybe the concentration effect; but there is a low evidence for me about that because I’m seeing this change of color in clear rough stones too.

About the hue change I just can say that I’m sure about the green hue to be compeletly abscent or present in different conditions, but as you see it’s not a dramatic change like green to red.

As I understand from the Atricle, Usambara Effect is due to absoption too, but elements like Cr+ in the stone’s formula do so and concentration effect mostly because of inclusions, right?

Although I saw that internal reflection make the same change of color but in rough stones with no internal reflection I see the same thing, also even candle light don’t chage the color of stone in comparison to white light and I test the stones with different lights and conditions and the only thing that for sure changes the color of the stones is the thickness that light goes through

But I should learn more about concentration effect.
Do you know any experiment to test these effects?

Appreciate your consideration