Monday, April 23, 2007

Color Change/Color Shift

Color change/color shift; whats the diff?

More on Topaz prices:

By Richard W. Wise

© 2007

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 Ural Mountains in the 1850s the standards were noon daylight and the flame of a candle. As we passed into the 20th Century the candle was replaced by the light bulb. This change passed unnoticed and led to some confusion and to the prevailing myth that alexandrite changes color from ruby red to emerald green. At 1500 degrees Kelvin, candlelight is distinctly reddish whereas the light emitted from a standard light bulb at 3200 Kelvin is well into the yellow. Thus, an alexandrite viewed by candlelight will appear to be much redder than that same stone viewed under the light bulb. Fact is the night time color of alexandrite regardless of source has always been purplish-red to reddish purple or as the great German gemologist Max Bauer described it in 1904: “an emerald by day and an amethyst by night.”

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 noon 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:

  1. Gems that shift part way between two adjacent hues. For example a pink sapphire that shifts from violetish pink to pinkish violet.
  2. Gems that shift from one adjacent hue to another. Sapphires that change from purple to blue are a good example.
  3. 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 Madagascar and East Africa. Don’t mistake this for abundance, color changing and color shifting gems are fairly rare. Several years ago I picked though literally thousands of rhodolite garnets to find just three that shifted from pinkish-purple to purplish-pink. Garnets with the alexandrite like color change are even rarer.

More on Topaz Prices:

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!



Interested in reading more about real life adventures in the gem trade? Follow me on gem buying adventures in the exotic entrepots of Burma and East Africa. Visit the gem fields of Austrailia and Brazil. 120 carefully selected photographs showing examples of the highest quality gems to educate the eye, including several of the world's most famous gems. Consider my book: Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones.

“Wise is a renowned author... He’s
done a marvelous job of this first book, monumental work, a tour de force...My recommendation: Buy this book”.

Charles Lewton-Brain, Orchid

Only $39.95. Read a couple of chapters online: www.secretsofthegemtrade.com.

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4 comments:

jasusang said...

Mr. Wise,

Thank you for posting the answer to my inquiry concerning color change/shift in gemstones. I appreciate your response since I don't have the time to post on the web forums working graveyard hours (midnight until 0800) in admin at the local PD.

Today's internet search gives responses on trying to sell you crappy gemstones from EBay instead of information, even though very few of the gems for sale are from reputable sellers. I used to post on the ISG forums, but, rarely would anyone respond and I have read several posts concerning color change/shift on the gemology online forums in which none made any sense. Ms. Barbara Smigel, of Artistic Colored Stones, is always very professional and helpful in answering questions for gemstone enthusiasts which she posts tons of information on her website at http://www.acstones.com. She has answered and posted several of my silly inquiries on her site.

I posted a response on GemWise concerning precious/semi-precious gemstones in hopes of winning a free copy of your book. I have only read the chapters from the book posted on the internet for pofolks. I assure you I will purchase one soon though.

The color shift garnet from Kenya is posted in incadescent lighting while the magenta color and in fluorescent lighting while the dusty rose color. The stone is posted as a "color change garnet" from Kenya.

I often visit Gemval for prices on colors/ct wt/cut on gemstones of so-called VVS clarity for a guideline of what sellers are asking on a monthly average for gemstones. For me, Gemval is a great website and tool for gemjunkies on if a gem you are interested in purchasing is reasonably priced according to an average of what gemstone sellers are asking for a particular gem of a certain color/ct/cut. But to find out prices for certain stones, such as tsavorite or spessartite, you have to be a member of Gemval.

You mentioned you have color change garnets for sell, which I would love to see. I often visit your website to view the garnets you have for sell. The demantoid is a real beauty! But I seriously doubt I can afford any of the garnets you have for sell. But, I enjoy looking at them.

Again, Mr. Wise, thank you for taking the time to explain to me the difference between color change and color shift.

Jan G

Richard W. Wise said...

Jan,

Thanks for the post. When GemVal was first introduced I was given a free short term membership. I logged on and did a few calculations and found the site more confusing than useful.

Frankly, given the large number of variables in grading a gemstone, my advice, read my book and educate your eye, there is no shortcut to connoisseurship.

RW

B. Goldman said...

Hi Richard,

When GemVal started to publish pricing analysis in 2005 I also found this service too rough. But since 2006 they reworked algoritms and the engine, and now it becomes much more accurate. now they publish some useful history chars. My opinion: actually GemVal is the best pricing info source on the web. I advise you to try it again.

Anonymous said...

Hi Richard,

I follow your explanation of color shift, but am starting confuse the semantics of various writers. In G&G Manson and Stockton garnet paper (W84), they define color shift as a change in color caused by transmited light vs internally reflected light without a change in illumination type. Fluorescent light produced the most dramatic change in pyrope-spessartite garnet when observed from the two perspectives (purple to green). What really matters to the consumer is the face-up change - reflected light of daylight vs incandescent type - in which case they show in thier example a change from purple to red much like the picture you have here (those aren't sophisticated color designations on my part - I defer to you). They refer to the transmitted light as revealing the body color. Somewhere I read that the GIA lab defines color-shift differently than this now.

I am intrigued by color-change phenomenon. While I understand the science of the underlying causes, the jargon baffles me. Some very interesting theories have been put forth by A. Halvorsen (author of 3 English papers on Usambara effect).

My pet rock is a garnet which changes from greenish blue to reddish purple - I can loose time making it do its thing.

I believe that if I could master your knowledge of color description I would Take a Giant Leap.

Your fan in Ithaca