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 Post subject: Colour Change Garnet, or just different light?
PostPosted: Thu Jul 09, 2020 10:44 am 
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I've been wondering about colour change. I can't figure out what constitutes a colour change rather than just a different light changing the appearance of the stones. Are there standards that define "Colour Change"?

Here are 4 photos of 3 small green garnets. The 2 on the left might qualify as colour change stones, maybe.

Sorry about the incandescent light. I looked all over the house for a large incandescent light and I only have the small flashlight I use in my gem tools. The 2 left stones turn a grey/blue under incandescent light. There is almost no change in the third stone.

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OUTSIDE in bright shade.

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LED lamp

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HALOGEN

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Small INCANDESCENT FLASHLIGHT

I only adjusted the brightness to be able to compare.


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 Post subject: Re: Colour Change Garnet, or just different light?
PostPosted: Thu Jul 09, 2020 1:09 pm 
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I would call them color change.
Here is what Vincent Pardieu said on the subject some years ago on our forum:
Vincent Pardieu wrote:
Hi,
While I was studying in Burma I had the same question regarding Burmese sapphires, that can be sometimes color change or color schift. In fact I was answered the following (and I still use it in this way): Color shift is when a gemstone present different colors using different light as Blue and purple, Color change is the same but the 2 colors are not adjacent on the color wheel (like green and purple). Some even people use the term "true color change" when the color on a different light is the complementary color on the color wheel (Like green and red)...

Anyway there are gemstone that present a color under day light and fluorewscent light, and then an other one under incandescent light. This is the typical situation but there are also some stones that present a color on day light, an other one under incandescant and a third under fluorescent light...
Color change is a very complex thing to study and one of my friends has already identified more than 30 gems presenting this phenomena and there are many different cases.

For him the terms color change and color schift are not enough to study the complexity of the thing even if you study only corundum...
All the best,

_________________
Vincent Pardieu


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 Post subject: Re: Colour Change Garnet, or just different light?
PostPosted: Wed Jul 15, 2020 6:02 pm 
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Barbra is the expert here.

But since this is still a somewhat subjective issue to this date (until some standards are better forced into the industry) ; I will give my personal opinion and say the two left stones are very slightly color shifters, and the right one no color change.
Vincent explanation is no surprise pretty much on the money. The subjectivity starts when deciding how much is the hue changing and when is it considered a shift or a change in color. There are several papers from GIA on this. Using something like the CIELAB colorWheel one can measure the change between the colors (usually taken from a D65 and A illuminant, skylight/incandescent illuminants that is). There was a proposal to say that a color Shift needs at least a 25 Degrees+ difference between the colors. A Color Change a 45degree+ and a true color change a 90degree+. This is the part that needs to be standardize!
Don't forget that the starting colors (usually ploted in LAB space) have to be from the resulting gem color under the different Illuminants and they have to be whitebalanced. In your photos the white is not balanced, so the paper is looking blueish in one and yellow in the other, This need to be balance out to judge the real color shift of the objects in question. Once you balance so the paper matches in hue between the two photos (can do this in a paint or photo software like photoshop) you will notice the gemstones you have will barely change color, hence what I mentioned above about a very slight color Shift.

If you want the nitty gritty
read this
https://www.gia.edu/gems-gemology/winte ... c-analysis


Last edited by TheL on Sat Jul 18, 2020 8:43 am, edited 1 time in total.

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 Post subject: Re: Colour Change Garnet, or just different light?
PostPosted: Wed Jul 15, 2020 9:13 pm 
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And here is a even more in depth paper on the matter

https://gem-a.com/images/Documents/JoG/ ... 31_5-8.pdf


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 Post subject: Re: Colour Change Garnet, or just different light?
PostPosted: Mon Aug 10, 2020 12:15 pm 
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TheL wrote:
And here is a even more in depth paper on the matter

https://gem-a.com/images/Documents/JoG/ ... 31_5-8.pdf


Thanks, I will look at the material and white balance the photos.
Christine


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