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 20^th 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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Lighting and Grading Gemstones Part II

LIGHT UP YOUR LIFE

by Richard W. Wise, G.G.
© 2006

What is the best lighting environment to view gemstones? In part I of this post I discussed the various lighting options and the pros and cons of each. In part II I will write about the lighting some of the world’s foremost gem experts and connoisseurs choose to both view and evaluate gemstones.

Among the experts opinions differ somewhat as to which “daylight” Kelvin temperature is best but each accepts daylight as the standard. Stephen Hofer one of the world’s most respected authorities on colored diamonds, maintains that 5500K works best for colored stones and 6500K is best for diamonds both white and colored. In his lab, which is dedicated to the evaluation of fancy color diamonds, all grading work is done under 6500 Kelvin fluorescents. C. R. Beesley, President of American Gem Labs in New York, prefers Vitalite a bulb manufactured by The Duro-Test Corporation with a Kelvin rating of 5500. "Most people don't do their homework", says Beesley, "comparing color temperature isn't enough..." Beesley tested more than twenty different light sources in the process of developing his Colorscan colored stone grading system to gauge their actual effect on gemstones.

In 1994 GIA completely revamped its color grading system for colored diamonds, shifting from Veralux, a so-called low UV lamp rated at 6200K, (see Secrets Of The Gem Trade, p.60, fn) which remains its standard for grading colorless diamonds to a 6500K average daylight fluorescent bulb for grading colored diamonds. The lamp is manufactured by Kollmorgen Corporation. GIA also uses a 6500K for general research purposes. The Institutes use of light of this Kelvin temperature is in no way an endorsement of the use of this type of lighting by jewelers. According to James Shigley, GIA's Director of Research, 6500 was chosen because it is a recognized standard used by color scientists. Little or no thought was given to enhancing the beauty of gemstones.

Incandescent and quartz halogen lamps can be made to resemble daylight with the addition of a blue filter. Duro-Test currently markets a "super-white halogen" that does the same job. I conducted a series of experiments with the Duro-Test product and found that this type of lighting does wonderful things for blue sapphire, aquamarine and amethyst, but does nothing to reduce the muddy look in blue and green tourmaline. "Super-white" induces a distinctly overblue look in rubies and tanzanite takes on an odd blue/violet multi-color effect quite dissimilar to its appearance in any other lighting environment. In diamond, "super-white" overly enhances the yellow/blue luster of the stone.

The problem!, filtered light makes for unintended results. Unfiltered fluorescent lighting can exhibit a relatively balanced spectrum when graphed on what scientists call a "Relative spectral power distribution" curve. Filtered lamps show spikes in this curve which are areas of color deficiency.

Another type of lighting called Neodymium is currently being marketed as a "full spectrum incandescent". According to Roger Schoenfeld, a lighting specialist with Durotest, this is really a standard yellowish incandescent incased in a special glass invented in Sweden, which reduces yellow and generates a spectrum richer in red and blue. Neodymium is not a bad choice. It is excellent for diamonds. Neodymium is a bit better than standard quartz for sapphire and aquamarine though not as good as "super-white". All three sources suppress the violetish to purplish secondary hue of the finer grades of sapphire.

Compared to daylight, neodymium works well with ruby, amethyst, emerald and tsavorite garnet. It is no worse with blue and green tourmaline which seems to be a true day stone, i.e. it looks its best in daylight and in daylight fluorescent 5000-6000K. Pink & red tourmalines do fine in Neodymium "full spectrum”, it does not produce the brownish secondary hue that most pinks exhibit in standard incandescent but, rather makes them look violetish. Tanzanite looks about the same as with quartz light; bluer than standard incandescent and without the otherworldly quality of "super-white".

Three or four years ago, Tailored Lighting introduced a 4700 Kelvin MR-16 lamp which has the highest Kelvin temperature of any incandescent bulb and may be the best solution yet for interior lighting. The new Solux lamp uses a new type of reflector to boost the quartz halogen lamp into the daylight range. This technology produces lighting with a fairly even power distribution curve (CRI) that shows no spikes in either the red or blue areas of the visible spectrum.

In a short, down and dirty test, Solux worked well with tourmaline, reducing the muddiness produced by all other incandescent light sources. Sapphire and ruby also benefited from this type of lamp, showing they’re colors in true daylight fashion. Solux also improved the diaphaneity of tanzanite. On the down side, Solux appeared to add a gray component to aquamarine and pink topaz which reduced the saturation and flattened the color.

Changing lighting environments have always been a problem for both buyers and sellers. You buy in one light, sell in another. Traditionally dealers who do extensive buying outside their offices have relied upon comparison stones, stones of well known color, which they carry or wear on buying trips.

I use two fixtures with twin four foot fluorescent lamps to give an overall daylight environment combined with several of the new Solux MR-16 4800K quartz halogen lamps in my own laboratory which doubles as a salesroom and consider this combination to be the closest possible to a true daylight environment. I use Duro-Test Vitalite in one fixture and Kollmorgen 6500K average daylight in the other. The use of the 6500K is to compensate for the 4800K Solux, to kick up the Kelvin temperature of the overall environment towards 5500K.

The combination of of daylight fluorescent with Solux works acceptably across the spectrum of gemstone colors. The fluorescents create an overall daylight environment and the Solux MR-16 provides the punch. This lighting temperature gives a balanced daylight color rendering when compared to New England north daylight. Several years ago, a German firm, System Eikhorst, introduced a lighting system based, in part, upon my recommendations. It includes both daylight fluorescent and Solux fixtures.

When making a purchasing decision it is important to identify the light source you are viewing the stone and to view the stone in as many lighting environments as can be found. Regardless of my geographic location at the time, I always compare each stone in daylight and 3200K incandescent to see how the stone reacts at both ends of the lighting spectrum.

If a consistent workable colored stone evaluation system is ever created, the lighting environment will of necessity be standardized. To achieve reproducible results, there are three variables; the observer, the gem observed and the lighting environment. The standardization of the lighting environment will remove one variable. This leaves one remaining variable, the observer. Either we must accept some nuances of subjectivity or build a gem grading robot replace to connoisseur’s eye and dictate our taste.

Light Up Your Life; Lighting and Grading Gemstones, Part I

LIGHT UP YOUR LIFE

Lighting and Grading Gemstones Part I

©2006

Richard W. Wise, G.G.

It used to be so simple, the brighter the light the better the light. You had the light from a fire and the light from the sun, that was it. In those good old days it was easy to figure out watt was watt! Today we have a myriad of options, “warm light”, “cool light”, “daylight.” Gem dealers are beginning to discover that the lights that make his rubies look like a pigeon's blood may make his tourmaline look like a dog's poop. As for the buyer, caveat emptor baby, check the bulb!

We are all familiar with the daylight standard: diamonds and colored gemstones are supposed to be judged in north daylight. Why?, because north daylight, specifically north daylight at noon is white light balanced between the red and blue spectrum. Sounds simple, but wait a moment! Dealers have long realized that the quality of daylight differs in locations throughout the world. North daylight at noon in Bangkok is qualitatively different from north daylight in New York.

The quality of natural daylight is affected by several factors, including latitude and air quality. In addition, the relative strength and color composition of daylight changes as the day progresses. "Don't buy blue sapphire after 2:00pm". That was the advice my Bangkok broker gave me on my first trip to Thailand. This dealer's truism teaches a basic fact that as the day progresses the color composition of sunlight moves from yellowish into the blue range then toward red at sundown.

Color scientists measure color as a function of light temperature and express it in units called Kelvin (K). a light filament heated red will have a Kelvin temperature of 1000-1500K, orange like a candle flame has a color temperature of 1500-2500K, yellow between 2500-4000K, white as in daylight is defined as a range; 4000-10,000K. In practice, increasing Kelvin temperature reduces yellow and adds blue.

If we wish to get a true color rendering using the daylight standard we will want a bulb that produces average noon sunlight with its relatively balanced color spectrum with a color temperature between 5500-6500 Kelvin. Unfortunately, the Kelvin temperature of an incandescent lamp falls between 3000-3200. The new low-volt quartz halogens have a color temperature of only about 3200K. Light in this temperature range is distinctly yellowish and will enhance the look of stones in the yellow, orange and red range. It will add an attractive bluish hue to chromium-vanadium colored gemstones like emerald, chrome tourmaline and tsavorite garnet. However, some Emerald, particularly Zambian emerald, can look distinctly overblue in this type lighting and 3000k light tends to muddy the crystal (reduce the transparency) of iron colored green and blue stones such as tourmaline and sapphire.

Daylight equivalent lighting does exist in fluorescents but not in incandescent single point spot and flood lighting that is used by most dealers and jewelers to bring out the brilliance of a gemstone. Daylight fluorescent bulbs with Kelvin temperatures between 5500-6500 are available. These lamps are marketed, usually as “daylight” lighting by several companies under various trade names. Unfortunately fluorescent lamps generate a diffused light and produce insufficient punch, what scientists call lumens, to bring out the best sparkle in your gems. To further complicate matters, there is no recognized international standard for a daylight bulb. Each manufacturer decides for himself what to consider “daylight”.

Coming next: Part Two, Learn what types of lighting the experts favor, Stay tuned...

So, you want to learn more about lighting and judging gemstones? Read the book:

"Secrets Of The Gem Trade is very highly recommended to anyone interested in gemology as a superbly organized, authoritative, comprehensive, and easy-to-follow reference."
Midwest Book Review
April 2006

www.secretsofthegemtrade.com