Category Archives: Gemstone lighting

Gem Cut Grading; Distinctions Without A Difference

A Non-Issue:

This is a good example of a non-issue. Non-symmetrical cuts, rectangle, oval, pear viewed under a single beam light source will always show brilliance across half the face of the gem when the stone is tilted away from the perpendicular. Why, because the gemstone is not symmetrical and therefore treats light in a non-symmetrical fashion. In the cuts just mentioned, some facets particularly the pavilion facets which are responsible for delivering brilliance, cannot be uniform. Some are larger, some are longer therefore light hitting these facets will refract in an irregular fashion. Symmetrical cuts, rounds and square cushions, by contrast have uniform facet patterns and do not suffer this phenomenon.

For this reason, asymmetrical gems are graded under a single light source, viewed at at a 45 degree angle away from the light source angled toward the eye. At this viewing angle, half the gem, the bottom half is potentially brilliant. The percentage of that half is then compared with the other half when the gem is rotated 180 degrees and viewed a second time. Add the relative percentage of each half and voila you have the total percentage of brilliance. In colored gemstones, 80% brilliance is considered excellent which means that at any given time, 20% of the gem will exhibit extinction.

One trick for viewing the brilliance of the entire stone at once is the use of sky light. Turn your back to the sun and view the gem at the same 35 degree angle toward the eye. In this position the light filtering around the body of the viewer should light up the 100% of the gem. Any lack of brilliance in this position is a fault and should be subtracted from the theoretical 100% to arrive at the percentage of brilliance. In most cases, gem photographers correct for this phenomenon by photographing gems with non-symmetrical outlines by using multiple light sources.

A Little Extinction Contributes To A Gem’s Beauty

Extinction is another much misunderstood phenomenon. I am often asked, particularly by members of this forum, if a particular gem shows extinction. First lets define our terms, what is extinction. This phenomenon is caused by off-axis refraction. When light enters a gemstone, it enters the crown and reflects internally off the pavilion (back) facets and eventually back through the crown to the eye. Inevitably some light refracts at an angle that is not toward but rather away from the eye, the greater the angle the more extinct it becomes, in a tonal continuum from gray to black.

All faceted gemstones without exception exhibit extinction, no extinction, no scintillation. the great German philosopher Hegel said determinatio est negatio. which when reduced to simplest terms means g all positive requires a negative. Sweet has no meaning without salty, good does not exist without evil, etcetera. So too with gems. Scintillation, sparkle is the result of light being broken up into pieces, tiny scintillas of light that are refracted back to the eye in little pieces. Between those pieces is darkness, extinction. Want a gem to light up like a flashlight with no sparkle, eliminate extinction. Some degree of extinction is therefore required as contrast.

Extinction As A Fault:

Extinction can have other causes as well, dark is the absence of light. Gems lacking transparency will often show what one of my clients once called a “heart of darkness” at the center of the gem beneath the table. This is particularly prevalent in sapphire and the cutter will often cut a window to let in light through the culet, the apex of the cone shaped pavilion of the gem. So while some extinction is desirable, in fact necessary to the beauty of the gem, large areas of extinction are a definite and definable fault.

Gem Grading: The Death of the Lightbulb and Other Brilliant Ideas

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Color temperatures a various types of lighting. 5500-6500 is the Kelvin temperature of daylight. GIA uses a 6200 Kelvin light source for diamond grading.

New Technologies May Require Changes is How We Look At Gems:

In 2007 amid little fanfare, Congress passed a law that required that the efficiency of that iconic household standby, the incandescent light bulb, be improved or perhaps accept its doom. The bulb has been around a long time and the technology has remained virtually unchanged since it was invented by Thomas Edison in 1881. Turns out the old bulb is a real energy waster, only 10% of the energy used is given off as light, the rest is dissipated as heat. Though some called it the death of the incandescent light bulb, Congress merely dictated an increase in efficiency, 20% by 2014, 60% by 2020.

Though the efficiency standards do not state what is allowed, such a dramatic increase in efficiency is bound to require new technologies which are likely to mean changes in the light spectrum produced by whatever technology replaces the old standard. No one has given much thought to the consequences this will have in the gem trade. The new standards are scheduled to take effect this January.

In the evaluation of quality, gemstones have been traditionally viewed under two light sources, noon daylight or more lately daylight equivalent fluorescent lighting and plain old incandescent (in the 19th Century it was candlelight). A stone that looked good by day but muddied up under the lightbulb is taken to be inferior to one that holds its color in both lighting environments. In 2003 I published a book, Secrets Of The Gem Trade, that divides gems into daystones and nightstones. The terms refer to gem varieties that look best under a given source. This seemingly bright idea may mean dramatic reductions in oil imports, but wait! What about the gem business, what’s a connoisseur to do?

The Tea Party To the Rescue:

Well our worries may be over, just last week the House passed legislation to deny funding to the law. Apparently the bill’s original Republican sponsor, Texas Representative Michael C. Burgess had an epiphany. He has seen the, ah, light. “The government has no right to tell me or any other citizen what type of bulb to use at home,” no matter how much energy it might save says Mr. Burgess. We have the right to waste all the energy we like in the privacy of our own bedrooms, says so in the scriptures.

But seriously folks! Sooner or later, new, more efficient types of lighting are bound to replace the old standby. Will there be a new standby? Probably not. We are pretty much at the point where the type of lighting used will be dictated by the setting that it is used in. Call it dial a light! At that point will gem grading light be standardized. To some degree it already has. Most laboratories use some artificial version of daylight. The Gemological Institute of America (GIA-GTL) uses a 6200 fluorescent bulb, American Gemological Labs uses a 5500 Kelvin bulb. What is the next step? Stay tuned to GemWise.

Book Review: The Colour of Paradise, The Emerald In The Age of Gunpowder Empires

Kris Lane

The Colour of Paradise, The Emerald In The Age of Gunpowder Empires

280 pages including appendices

ISBN: 978-0-300-16131-1

The history of the gem trade is a difficult research topic because gemstones are very small objects of great value that have been highly sought after for millenia by rich and powerful people looking for wealth that was portable and easily concealed. The trade itself has been controlled for centuries by minority groups, often oppressed minorities, Jews, Armenians and Indians for whom secrecy was a proven form of self preservation. Kris Lane is a historian, a Professor of History at the College of William and Mary. In The Colour of Paradise, Professor Lane focuses very well honed research skills on the history of the emerald, one of the rarest, most mysterious and highly valued of all gemstones.

The book contains no particularly major revelations. Most historians of the trade are aware that India’s so-called “old mine” emeralds were, in fact, Colombian emeralds imported by the Spanish into India in the Sixteenth and Seventeenth Centuries. Lane’s contribution is to meticulously document both the early history of Spain’s brutal exploitation of Colombia’s indigenous people and its gem wealth. He gives us a well documented overview of early trade routes and uncovers some very interesting and original information concerning 16th Century production; methods and emerald values.

Lane begins with the 16th Century and follows emerald production in Colombia right up to the present, with a good account of politics and production into the 1990s.

All and all this well organized and well written account brings real clarity to a relatively murky area of history. The book also contains detailed appendices estimating early emerald at Muzo, relative values of emerald and diamond in Europe in the 17th Century and an extensive bibliography. The author has uncovered several original accounts that have until now been unrecognized. Highly recommended.

Online Gem Evaluation; Slouching Toward Disaster, Part III

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by Richard W. Wise, G.G.

You Just Can’t Hide Those Lying Eyes:

Image A: Before photoshop, this is the image the camera saw. Photo Courtesy Precision Gem

In my first post on this subject, I made the point that making a purchasing decision or merely comparing prices by comparing images is fraught with difficulty. Photoshop and other similar programs are a good part of the reason why. The two images to the left illustrate the point. Image A is a custom cut Mali garnet. Based on the image, the stone appears to be a slightly greenish (10% yellow light toned garnet of medium saturation, not bright but not overly dull with a slight gray mask.

In Image B of the same stone, the original image has been altered by a simple tweak of the Saturation Enhancement Tool in Photoshop. The green secondary hue now appears more prominent, perhaps 15-20%. The big change, however is in the level of saturation. The color must now be described as vivid effectively doubling the value of the stone. The only hint that this is the same image is that the background in image A is slightly cooler (grayer) than the background in image B. However, if the photographer simply took two shots under the same lighting he could easily disguise that fact. Backgrounds can also be removed or replaced with a uniform black. Assuming that these two images appeared at similar prices on separate websites, an online bargain hunter would wrongly conclude that the owner of the stone labeled image B was offering his goods at a much more favorable price.

Approaching the Pinnacle of Perfection, Small bumps in Quality Equal Big Bumps in Price.

In low quality commercial grade gemstones, small differences is quality will not make a great difference in price. However, as quality nears theoretical perfection the importance of smaller differences is magnified. In the stratosphere of gemstone quality, small differences can make for very large price differentials. Lets take an example that everyone knows, the D flawless diamond. I chose this example because diamonds are very precisely graded using an internationally accepted grading system. According to The Guide a well respected industry publication, the current wholesale price of a 1.00 carat D-IF is 31% higher than the very next color grade, E-IF, compared by clarity grade, (D-Fl vs D-VVS1) the spread is slightly less, about 29%. A similar comparison between diamonds with a color grade of L and O shows only a few hundred dollars separating the two grades. These differences will not show up in an online image.

In colored gemstones these same percentages apply and the grading equation becomes much more complex. Colorless diamonds are graded based on slight tonal variations of yellow. Color, any color, breaks down into two additional factors, hue and saturation which must be added to tone in the quality equation. In nature there are few pure hues. The hue of a gemstone is composed of primary, secondary and sometimes tertiary components, a top color ruby for example, may be 75% red, 15% purple and 10% orange. A blue sapphire with 10% (or less) green secondary hue which will likely not be visible in an online image will sell at a dramatically lower price than a stone with a pure or slightly purplish blue hue. Similarly, an emerald that is 75% green with a 10-15% secondary blue hue can sell for double the price of a slightly yellowish stone. These slight differences in hue do not show up even in professional images. This makes online comparison between two images very deceptive and of little or no value.

Image B: The same gemstone after a tweak of the Saturation Enhancement tool in Photoshop

The Value of Images:

Online images are of some value. As a professional, I never use images to determine which stones I will buy though sometimes they can identify a stone that I will not buy. It is usually possible to tell something about the clarity and cut of a given gem by viewing the image. Both come with caveats.

Online Images are Many Times the Size of The Stone; so are the inclusions:

The Mali garnet at left weighs approximately one carat. That means the stone is somewhere in the range of 6 mm in diameter. The image shown is 38mm, 6.33 times larger. Many gems that are eye-flawless will appear visibly included (flawed). In colored gemstones, the eye standard replaces the loupe standard, what the eye sees is what is important, magnification doesn’t effect price. In most varieties of colored gemstones, the difference between eye-clean and visibly included is dramatic. It is similar to the difference between a diamond graded flawless and another graded SI2.

In some cases the image will actually distort what the eye sees. Award winning gem photographer, Robert Weldon, makes the point that due to the limits of depth of field, the camera’s lens will compress inclusions into a single plane increasing the prominence of the inclusions in the image. This compression can lead to particular difficulties when trying to accurately render images of expensive type II and III gemstones such as emerald where the difference in value between an eye-clean gem and one with eye visible inclusions will be dramatic. Online images are normally in j.peg format. This format is created, Weldon points out, by subtracting information in the original high resolution image.

Images can be useful in evaluating cut but bear in mind that the visual performance in a gem depends upon lighting. A well lighted gemstone may appear to perform better than one that is less well lit. The lighting environment is not visible, multiple light sources of the type normally used in photography can mask real deficiencies in cut.

Color and Lighting:

In the good old days, there were two types of light; the sun and light from a natural flame, a fire or a candle. This, as I point out elsewhere, is the source of the legendary “ruby red” alexandrite. When that gem was discovered in the mid nineteenth century, incandescent light was supplied by a candle and candlelight is distinctly reddish—after the invention of the light bulb, the ruby red alexandrite became distinctly purple because that light source is yellowish. With today’s technology, it is possible to virtually cherry pick a lighting environment that is strong in a given color. You can light sapphires with blue light, rubies with red. Again the light source is not visible—so how would you know until your gem arrives in the mail.

Online Gem Evaluation or Slouching Toward Disaster:

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by Richard W. Wise, G.G.,

author: Secrets Of The Gem Trade.

With the commercialization of the internet we have entered a new era for collectors of all stripes. Whether you are interested in antiquarian books or colored gemstones there is a great deal of information available and the opportunity to compare prices for the best deal.

If you are buying a commodity and I define commodities as items of uniform quality that are produced in multiples, comparisons are straight forward. Deciding between two new copies of the latest Jacky Collins romance novel is simply a question of price though you have to watch the shipping charge.

When it comes to collectibles, such as old coins where condition is a primary determinant of price, the buying decision becomes a bit more difficult. Two identical 1870 U. S. ten dollar gold pieces will vary dramatically depending upon condition.

Purchasing gemstones is much more complicated. First, no two gems are alike. Even colorless round brilliant cut diamonds that are traded with “certificates” that grade each of the four C’s vary in the relative percentages of brilliance, dispersion and scintillation. Even the so-called “ideal cuts” will vary markedly because the definition of what constitutes ideal varies from lab to lab.

Colored gemstones present an even broader set of variables and presents an almost insurmountable barrier to effective online comparison. In gemstones, color is the most important determinant of value and sub-divides into three factors, or values; hue, saturation and tone. Accurate grading of a gemstone depends upon two factors; the lighting that the gem is viewed in and the viewer. Change either and you no longer have consistent or accurate grading.

Online Color Comparison; The Impossible Dream:

The rise of internet forums has lead to the dubious practice of online grading by pixel. A neophyte gem buyer will post an image of a prospective purchase with a plea to the forum members to basically appraise the gem and by comparing quality with the purchase price, determine if said newby is getting a good deal. Unfortunately a number of forum members, many of whom ought to know better, jump in and tender their opinions. Though many online grading gurus would disagree, this sort of color comparison is nearly impossible.

There are a host of variables each of which will fundamentally alter the color that you think, you see. The image itself was taken by a specific camera in a specific lighting environment. Each make and model of camera has specific pluses and minus when it comes to accurately rendering color. Some are good with greens, some with blues others with reds. The color temperature of the specific lighting will make a big difference is the color you see. Natural light is reddish at dawn, turns yellow in the late morning, is neutral white at noon then turns bluish in the afternoon and finally reddish again at sunset. Artificial light can be controlled to emit almost any hue all of which affect the apparent color of gemstones.

Apparent color can also be altered by easily altered by editing software. Photoshop, the world’s most popular color editing software employs sophisticated tools that can easily alter the hue/saturation/tone of an image. With five minutes of training, the most unsophisticated Photoshop user can be taught to turn a ruby into an amethyst or even turn it green as an emerald. Though I have seen some forum gurus claim to be able to detect images that have been altered, such claims are pure nonsense.

Now we get to the monitors. Each monitor is different. 24 bit monitors differ markedly from 16 bits and from manufacturer to manufacturer and even day to day. Jennifer Robbins author of Learning Web Design tells her readers: “Let go of precise color control. Yes, once again, the best practice is to acknowledge that the colors you pick won’t look the same to everyone, and live with it. Precise color is not a priority in this medium where the colors you see can change based on the platform, monitor bit-depth, or even the angle of the laptop screen.”

No Computer Monitor on Earth…

$On one popular forum this spessartite garnet, offered by a Thai vendor, was said to show brown. Anyone with experience photographing this gem variety knows that the camera will often show brown where there is none. A result of off axis refraction.$

On one popular forum this spessartite garnet, offered by a Thai vendor, was said be brown or brownish. Though all my monitor shows is deep orange, those with experience photographing this gem variety know that the camera will often show brown where none exists. This is a result of lighting and off axis refraction.

The problem in accepting Ms. Robbin’s advise is that precise color is a priority nay a necessity in color grading gemstones. The Colorscan grading system pioneered by C. R. Beasley, founder of American Gemological Laboratories (AGL), the only U. S. lab that quality grades colored stones uses 36 different hue/tone combinations together with two degrees of gray masks yielding 108 different colors each associated with a specific quality level and therefore price, in blue sapphire. No color monitor on earth, even if properly calibrated can render all these combinations. The current director, Gemologist Christopher Smith, uses sample gemstones for stone to stone comparisons, he says he has literally hundreds of sample stones.

Viewed face-up, a gemstone is a mosaic of color, it is a complex scene. A static image captures but one aspect of a gem under a specific lighting environment. Many factors will effect the image you see. Lighting, viewing angle, lens and angles of refraction. The images shown here illustrate some of the problems.

I’ve chosen to discuss brown in orange gemstones because brown is dark toned orange. This means that the color you see is particularly vulnerable to misinterpretation. The gemstone acts a a prism, bending and bouncing light about like a demented pinball. If the light ray refracts at an angle from the perpendicular, off axis, the hue will appear darker, if totally away from the eye, black.

This professionally taken image of a 3.29 carat Nigerian spessartite from my inventory is almost identical in color (hue/saturation/tone) with the 7.46 carat stone at left yet it appears on my monitor as distinctly more orange and 15% darker in tone. Photo: Jeff Scovil

This 7.46 carat spessartite shows visible brown. It is from my own inventory so I can state with certainty that the stone itself has absolutely no brown.

In the image, this 7.46 carat spessartite shows a distinct visible brown. It is from my own inventory so I can state with certainty that the stone is a rich, vivid pure orange with absolutely no brown secondary hue or mask. On my monitor, the stone the image appears distinctly yellowish with brownish scintillation. Compare to the image of the 3.29 carat stone at right. Photo: Jeff Scovil

The Nuances of Grading:

Few jewelers and fewer collectors ever have the opportunity to see the complete range of quality possible in any given gem variety, be it sapphire, ruby, emerald or spessartite. Without that overview a grader lacks context. As the quality of a given gem approaches perfection, the beauty and rarity and therefore the price of a gemstone will increase dramatically. In the next post I will discuss the subtle nuances of grading that define those gems that are truly transcendent. Stay Tuned or sign up. You can receive GemWise by email or RSS feed, don’t miss another gonzo post, look along the right side and sign up ! I don’t guarantee that you will always agree but I promise it will never be boring. Comments welcome!

Color Change/Color Shift

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Lighting and Grading Gemstones Part II

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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 stan
dard 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

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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