Color change/color shift; whats the diff?
More on Topaz prices:
By Richard W. Wise
I Get Emails:
I love hearing from readers. I get numerous emails every week from all over the world mostly from folks have read Secrets Of The Gem Trade. Some of you write asking detailed questions, often already answered in my book, which would require much more time than I have available to answer. Others send images of gems they are considering buying. My apologies but if you want my professional opinion about a purchase, I am going to charge you a professional fee and I don’t work from images.
The forums are an ideal place to ask these sorts of questions: For questions on gems; log onto www.gemologyonline.com. These folks love difficult questions, the more difficult the better. For questions about pearls I recommend: www.pearl-guide.com
Color Change versus Color Shift Gems:
Just the other day, Jan Glover wrote asking a stimulating question.
“First, would you please try to explain the difference between a color shift and a color change gemstone to me? I have seen garnets posted on the internet that change color from a purple to a pink in different types of lighting, is this (pictured above) a color shift garnet?”
Great question Jan!
First a little color theory, consider the color wheel. Actually, there are several color wheels, the one we are interested in is the spectral color wheel, that’s the one that divides white light into its constituent colors or hues. To that wheel we make a couple of additions. The eight chromatic hues present in gemstones are; red, orange yellow, green, blue violet, purple and pink. The first six are the spectral hues, the last two; purple, a hue that lies half way between red and blue and pink (pale red) are known as modified spectral hues. If we were using terminology rigorously as we should, we would not even use the term color. Properly speaking; the terms should be; hue shift and hue change
Gems change color when the lighting environment changes. In the old days there were just two types of light; sunlight and the light from a flame. When alexandrite was first discovered in the
In 20-21st Century gemology, the two types of lighting used to judge color change in a gemstone are sunlight, specifically north daylight at noon and incandescent or light from a standard light bulb. North daylight at is fairly balanced white light between 5500-6500 Kelvin. Standard incandescent is yellowish light with a Kelvin temperature of 3200. Specificity is crucial; in today's technological world it is possible to dial-a -hue, light sources are available with Kelvin temperatures that cover practically the entire color spectrum. Caveat emptor!!
Many gemstones will exhibit alterations of hue when the lighting environment is changed as specified above. There are three possibilities:
- Gems that shift part way between two adjacent hues. For example a pink sapphire that shifts from violetish pink to pinkish violet.
- Gems that shift from one adjacent hue to another. Sapphires that change from purple to blue are a good example.
- Gems that leap across the color wheel from one (non-adjacent) hue to another. For example, alexandrite and some garnet that change from purple-red (P-R) to blue-green. (B-G).
In my opinion, no. 1 should be called a color shift and cases numbered 2 and 3 should properly be termed color change. The first case is a change in degree not in kind, the hues shift only partially from one hue to another, never completely. In the second and third cases there is a change in kind not just degree. In both 2 and 3 we see a true change of hue. I have seen certain Sri Lankan sapphires change from a slightly grayish blue to a true purple and, of course, there is the example of alexandrite and certain alexandrite like garnets.
As to your question Jan, impossible to say. First question what light source was used in capturing that image? At a guess I would say that we are looking at a rhodolite color change stone. I have found one or two in parcels in East Africa over the years. Given my classification, the stone would fall into category no. 1 and be classified as a color shift garnet.
Recently, there have been quite a number of color change and color shift garnets, some in fairly large sizes, coming out of
Somebody mis-spoke, probably me. What I meant to report in my last post was that topaz prices in Brazil had increased dramatically over the past several years. In a recent post on www.yourgemologist.com , New York dealer Steve Lembeck takes issue with the figures reported in this blog. David Epstein agrees with Steve Lembeck. According to Epstein prices at this year's Tucson shows were up between 20-40%; peach up 20%, peachy/pink up 25%, pink up 25%, sherries up 30% and reds up 40%.
Epstein continues to maintain that prices, based on actual transactions in Teofilo Otoni, have increased 200-300% but this increase has been over a period of three and one half years. Hopefully that clears up my confusion, mea culpa!
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