Buying and Selling Gems: What Light is Best? - Part 1
Buying and Selling Gems:
What Light is Best?
PART I: NATURAL LIGHT
By William J. Sersen, Ph.D., AG (AIGS) and Corrine Hopkins, FGA, AG (AIGS)
Formerly of the Asian Institute of Gemological Sciences, Bangkok, Thailand
Note: This article is reprinted with permission. It originally appeared in the Gemological Digest, Vol. 2, No. 4, 1989, pp. 13–23. To read Part II of this article, click here
Natural light has traditionally been considered the standard light source for buying and selling gems. But is it really a standard? In Part I of this two-part article, dealers of various nationalities were asked to describe their experiences with natural light as it relates to rubies and blue sapphires. Their responses produced a host of surprises, not the least of which was that few dealers agreed as to just how natural light affects a stone’s appearance.
This prompted AIGS to do its own study on selected rubies and blue sapphires. Results of both the dealer poll and the AIGS study appear here. More importantly, the variations of natural light are accurately cataloged, with tables provided that will allow dealers in many places around the world to determine what type of natural light is best suited to their buying and selling.
Part II of this article will appear in a future issue, and will discuss the suitability and use of artificial light sources.
In the business of buying and selling precious stones there are a number of little tricks picked up along the way, tricks that often make the difference between profit and loss. Collectively, we might call them “experience,” for that is normally how they are acquired and the price is, usually, high. “Experience” of this sort is not found in gemological texts; it comes only through hard knocks—i.e., buying stones from someone who has a bigger box of experience than your own—or, via a bit of friendly advice passed on by one who has been there before. Most dealers have a collection of this “experience,” kept in a box at the back of the safe or in some dusty drawer. It amounts to the small pile of gems which are unsalable; the gems you have learned valuable lessons by buying. In other words, gems you should not have bought in the first place.
One bit of experience that every stone dealer worth his rocks soon acquires is that a stone’s appearance is not constant. Instead, it can and often does change with the quality of light under which it is viewed. And a change in color appearance often means a change in value.
In the days before electric light sources, traders could only view their prospective purchases under natural light or by the light of a candle. Natural light means direct sunlight and skylight (light coming from all directions of the sky except directly from the sun).
Some dealers would examine a stone at various times of the day, realizing that the position of the sun in the sky, together with weather conditions, affected overall color appearance; others took it one step further, viewing the gem in sunlight, skylight and in the shade of a tree in order to get an idea of how it would look in any lighting situation. Similar practices continue to this day, despite the availability of artificial lights of various kinds.
So, why examine gems under natural light, the quality of which is subject to a myriad of changing weather conditions, when our Modern Age offers us incandescent and fluorescent (including simulated daylight) lighting? For that matter, why bother to view a gem under more than one light source, be it natural outdoor light at a given time(s) of the day vs. the stone’s appearance in the shade, or in fluorescent simulated daylight vs. incandescent lighting? The answers to those questions lie in whether you are an astute buyer or seller, and in what part of the world you happen to be conducting business.
“One bit of experience that every stone dealer worth his rocks soon acquires is that a stone’s appearance is not constant. Instead, it can and often does change with the quality of light under which it is viewed. And a change in color appearance often means a change in value.”
Viewing gems under natural light
As in other parts of the world, it is common practice in Thailand for dealers and professional buyers to view colored stones at a table situated at a window. Natural light is the accepted lighting “standard,” some dealers and buyers preferring north skylight only.
In the days before the GIA Diamondlite, such was also the case internationally with the color grading of diamonds.
Writing in 1916, Frank B. Wade notes in his classic volume on diamonds:
In the first place see that you have a good north light, unobstructed by buildings or other objects. There must not be any coloured surface near by to reflect tinted light, as a false estimate might easily result.“In the second place, do not attempt to judge stones at all closely except in the middle of the day, say between 10 A.M. and 2 P.M. Very erroneous results may easily be had by neglecting this precaution.“Dark or dull days should be avoided also. One must have plenty of good neutral light to make fine comparisons.
What Mr. Wade has to say about north light and (especially) weather conditions is echoed by many local colored-stone traders today. However, the authors of this article were particularly struck by the comment “between 10 A.M. and 2 P.M.” as it is reminiscent of remarks heard in Thailand and Burma about rubies and blue sapphires looking “better” or “worse” at different times of day.
So, wondered the authors, does the quality of natural light vary enough to cause noticeable differences in the appearance of rubies and blue sapphires at different times? Not only had Bangkok dealers mentioned this before, but some had gone so far as to say that they regulate their buying and selling according to the time of day and weather conditions.
It was decided to telephone a few local colored-stone dealers, all of whom have been in the trade for years, and ask them the following questions:
Do rubies and/or blue sapphires change appearance at different times of the day? If yes, when do rubies look better/worse? When do blue sapphires look better/worse?
All agreed that the color appearance of rubies and blue sapphires changes in the course of a day, and all specified what times those stones look best/worst. But, to the authors’ astonishment, there was no consensus as to what those times are. This was all the more interesting in that a few stated that they try to coordinate their buying/ selling of these stones with the time of day in which the color appearance was best (= selling) or worst (= buying).
Spurred on by curiosity, the authors and two other AIGS staff carried out their own experiments. A selection of rubies and blue sapphires of mixed “type categories” (see Sersen, 1988) was periodically examined for two weeks. North and east window lighting was used. Weather conditions during this period ranged from bright and sunny to dark and rainy. The purpose of these experiments was, of course, to see if the stones would change at all in color appearance.
The gems were viewed four times daily. Hue, lightness and saturation was recorded on each occasion, together with respective weather conditions. These notations were based strictly on visual observation. No conclusive results were had, possibly because no comparison reference was used; only the testers’ memories were involved, just like with most dealers.
Now more curious than ever, the authors took a formal written survey of 20 colored-stone traders in order to compare their answers and see what patterns, if any, might emerge.
The questions asked concerned the lighting conditions used for buying and selling, whether rubies/blue sapphires change color appearance at different times of the day (and if so, when do they look best/worst) and specifically what factors are thought responsible for color appearance changes when such changes are seen.
All 20 traders were interviewed in Bangkok. They consisted of 9 Thais, 6 Americans, 3 Burmese, 1 Canadian and 1 Malaysian. The majority are local wholesalers and sales personnel for local wholesalers. The others consist of Thailand-based brokers and overseas-based dealers who buy in Thailand and/or Sri Lanka and sell in Europe and/or America. The trade experience of those questioned ranged from 2 to 50 years, with most having at least 10 years experience. Every attempt was made not to phrase questions in a leading way. People were simply asked questions and encouraged to “talk on” for as long as they wanted, without prejudicing comments from the interviewer.
“All agreed that the color appearance of rubies and blue sapphires changes in the course of a day, and all specified what times those stones look best/worst. But, to the authors’ astonishment, there was no consensus as to what those times are.”
What lighting do you use when buying stones?
Most people (75%) said they buy ruby and sapphire after examining those stones under natural skylight only. Of those, seven people prefer north or northwest skylight, seven use any direction of skylight, one specified north or south skylight and one south skylight only. Of the remaining 20%, one buys only after viewing each stone under north skylight and direct sunlight; one uses north skylight or a “daylight lamp”; two view their prospective purchases under multiple natural and artificial lighting conditions; one said he buys using “whatever lighting arrangement happens to be available.”
Lighting used when selling stones?
The majority (55%) of those questioned said they use skylight for selling as well as buying. Several stated categorically that the color appearance of rubies and sapphires changes with the time of day and they therefore prefer to buy in “bad light” and sell in “good light.” The rational behind this is that if the stone appears reasonably nice under less complimentary lighting, it will look good under any (natural) lighting. Selling in “good light” means exactly what it implies: during times when natural lighting conditions make the gem look best.
One dealer said he buys in Sri Lanka using only northwest skylight, and sells in his U.S. office under quartz halogen lighting. Another stated she buys rubies under north skylight, but prefers selling them under “direct sunlight in the afternoon, because the light is yellow.” In both instances, the lighting used for selling is perceived as complimentary to the gem’s color appearance.
The rest largely buy in skylight and sell in whatever lighting is available or under lighting conditions expressly requested by a customer, such as skylight from a particular window direction. Obviously, dealers who do all their buying and selling from one office location have more control over lighting conditions than does a broker who must sell—and accept consignments—under whatever lighting is available, natural or artificial.
Do rubies and/or blue sapphires change appearance at different times of the day?
This question solicited the most interesting answers. Ninety-five percent of those polled replied with a firm “Yes.” Thirteen said rubies tend to look best only at certain times of the day. Seven said the same for blue sapphires. Conversely, nine people remarked that rubies often appear less beautiful at definite times of the day, with 11 saying the same of the sapphires.
Though many had firm opinions on this matter, there was little agreement on what times these stones look better or worse! For example, some said rubies and blue sapphires look the best at specific times of the morning, though others said mid- or late afternoon. Two dealers noted that rubies in particular look best towards sunrise and sunset.
Those who did not cite specific times when these stones look “better” or “worse” often associated apparent color appearance changes with weather conditions. For instance, five people observed that blue sapphires look best “at any time the sky is clear.” Two said the same of ruby. However, a third stated emphatically that rubies look best “only when the sky is slightly cloudy, because clear blue skies give them a purplish tint.”
“Though many had firm opinions on this matter, there was little agreement on what times these stones look better or worse! For example, some said rubies and blue sapphires look the best at specific times of the morning, though others said mid- or late afternoon. Two dealers noted that rubies in particular look best towards sunrise and sunset.”
And, yes, several people volunteered that they prefer buying and selling ruby and/or sapphire in strict accordance with the times and/or weather conditions they cited.
Why do these changes in color appearance occur?
Some said they did not know. Others ascribed the changes to weather conditions or the “quality of sunlight” at different times of the day. A few said it all depends on the relative position of the sun in the sky-and the corresponding light intensity.
When asked specifically if light intensity affects the beauty of a ruby and/or blue sapphire, 95% said “Yes.” Six dealers added that light blue sapphires look best under dimmer lighting (‘toward the evening or whenever the sky is cloudy’). Dark sapphires and rubies, said several, look best when the light is comparatively bright (‘around midday or noon’).
A few said that inclusions are more visible in lighter rubies or sapphires when those stones are viewed under intense natural light. As one wholesaler put it, “their nakedness is exposed.” 1
It was evident when talking to those surveyed that if lighting conditions are not optimum—for instance, the presence of a dark cloudy sky—many will not show stones to buyers. In the Bangkok gem trade, the general rule for natural light is “the brighter the better.” This is partly why dealers here prefer natural over artificial light; that is, natural light is more intense than that of an “artificial diffused daylight source.” Hence, it is often more complimentary to stones.2
Viewing gems under natural light in different countries
The following story is probably familiar to many of you:
So-and-so bought a blue sapphire in Bangkok or Sri Lanka. He brought it home to America, where it seemed to “look different.” So-and-so thought to himself, “Is this the same stone? It doesn’t look as nice as it did when I bought it. Did those Asian dealers switch the stone on me? Maybe they did, because it looks different!”
This is a common story. The authors have heard it from a number of people (including local dealers and overseas buyers) over the years. This story arises because stones can and do assume slightly different color appearances in different latitudes. As we shall see, this is partly ascribable to regional variations in atmospheric moisture, dust and pollutants affecting the color quality of the light.
To quote an example given by one local dealer, “any ruby or blue sapphire that looks dark in Thailand will look darker in Europe or America.”
Another dealer (an American who frequents Bangkok, not included in the survey above) had once mailed stones to customers first in Los Angeles, California, and thereafter Montana. Reactions he received–concerning the same stones—indicated that those stones appeared differently to each party under natural light. This may come as no surprise to anyone who has seen the skies of pollution-choked Los Angeles vs. the clear blue Montana skylight.
Benjamin Zucker (1976) has pertinent comments on this same subject:
One of the gem dealer’s distinctive talents must be to add or subtract in his mind portions of the color and brilliance that he sees, so he can make allowances for being in Amsterdam, in India, or inside a New York retail establishment with incandescent lighting.“A June afternoon in Bombay… will emit a light so overpoweringly bright that the ruby will take on a magnificent deep red color with a vibrant cast of brilliance.… If the same stone is shipped to New York and examined in natural New York daylight, it will appear a considerably darker shade of red and less brilliant. This is due to the high amount of pollution over New York and to the fact that New York lies farther from the equator than Bombay. But Amsterdam light is even grayer than New York daylight!
A scientific basis?
So, why do gems appear different under natural light depending on the latitude, weather conditions, and time of the day? For that matter, do they appreciably change in appearance, or is this largely a product of imagination?Though imagination and the fallibility of one’s color memory probably play a role, there is ample scientific support for general claims of color appearance change. In order to appreciate this point, we must first define skylight, sunlight and their relationship to the principles of color temperature and the scattering of light.
Skylight, or, why is the sky blue?
As mentioned before, skylight is the light we perceive when looking away from the sun at the sky. Sunlight is the light observed when looking directly at the sun. Daylight may be thought of as a combination of skylight and sunlight.
Sunlight and skylight differ in appearance. On a clear day when the sun is overhead, sunlight appears whitish. As the afternoon progresses, it becomes increasingly yellow, then, depending on atmospheric conditions, orange, and finally, just before sunset, red. This shift in color is described in terms of color temperature, which is measured on the Kelvin scale. Thus, the color temperature of sunlight at sunrise or sunset is about 1,800° Kelvin, signifying the predomination of red wavelengths. Temperatures of 5000° K to 5500° K denote a more even distribution of all wavelengths, resulting in the whitish appearance the sun assumes at noon.
Skylight ranges from very pale blue to deep blue. The purity and saturation of the blue is influenced by atmospheric moisture, dust and pollution. Generally speaking, the sky is palest when the atmosphere is humid or laden with dust (Figures 3 and 10); it is deepest blue when the air is dry and free of pollutants (Figures 1 and 2). This corresponds to color temperatures of about 6500° K to well over 20,000° K, respectively, the higher Kelvin temperatures indicating predomination of the blue-violet wavelengths.
The color quality of the sky varies with latitude, partly because different latitudes have correspondingly different weather patterns. Countries with drier air tend to have deeper blue skies; those with more atmospheric moisture tend to have paler skies (Mueller & Rudolph, 1972).
Therefore, the quality of skylight at any given latitude depends on a complex interaction of sunlight with the local presence of atmospheric moisture, dust and pollutants.
Specifically, it is the scattering of light that causes the blue color of the sky and the shifts in color temperature of the skylight and direct sunlight.
The word scatter literally means “to cause to separate widely.” In this context small particles of matter, such as air molecules and dust, cause the individual photons of impinging sunlight beams to deflect (scatter) in all directions. As sunlight enters the earth’s atmosphere, violet and blue wavelengths are scattered the most, followed by green, yellow, orange and red, in that order. Shorter wavelengths (violet, blue) are scattered about ten times more so than the longer red wavelengths (White, 1959).
To visualize this phenomenon, imagine a ray of white light (the composite of all spectral hues) passing through a cloud of smoke. The blue portion scatters off in all directions. With much of the blue light eliminated, the beam appears more yellow as it exits the cloud. An observer (Observer A in Figure 5) looking directly at the beam as it leaves the cloud will notice that it has a yellowish color. Someone viewing the cloud from any other direction will see scattered bluish light (Observer B in Figure 5).
Similarly, if our observer looks directly at the sun, he perceives it as one color (whitish, yellow, orange or red, relative to atmospheric conditions and the sun’s position in the sky), though the sky itself appears blue because of the scattering effect (Figure 6). When the sun is overhead and the weather is clear, the entire sky appears light blue. This is the composite color blend of the scattered wavelengths, the additive mixture of violet, blue, green and yellow.
As the sun continues on its westward path, increased scattering takes place till, shortly before sundown, most of the blue and violet wavelengths are “scattered out.” Hence, a stationary observer watching the sky from noon till late afternoon may observe a gradual increase in the saturation of the blue sky: to wit, an increase in color temperature which can potentially span many hundreds, and even thousands of degrees Kelvin. This is not readily observable when the sky is overcast, laden with dust or strongly polluted.
“Specifically, it is the scattering of light that causes the blue color of the sky and the shifts in color temperature of the skylight and direct sunlight.”
The Purkinje shift
One day, many years ago, a man named Johannes von Purkinje went walking in the fields at dawn. He observed that blue flowers looked brighter than red flowers. Later that day, when the sun was overhead, the red flowers looked brighter than the blue ones. Conclusion? The human eye is more sensitive to blue when the light is dim and red when the light is strong. This phenomenon is called the Purkinje shift. Because of it, flowers that are bright red on a sunny afternoon look bluish-red toward evening (Varley, 1980).
The eye’s perception of visible light under varying light intensities is illustrated by the spectral sensitivity curve below. Note the shift in sensitivity toward the blue-violet end of the spectrum when lighting is dim.
The bottom line
Now that we have most of the science out of the way we can get down to the million dollar question. Just how does all this affect the color appearance of rubies and blue sapphires? In a nutshell, blue sapphires may tend to “look better” in the late afternoon or early morning when viewed under skylight. With increased scattering of shorter wavelengths during those periods, the skylight itself is more blue. The effect on the stone is comparable to that of shining a blue light on a white egg: the egg appears blue. Additionally, the eye is more sensitive to blue during these same periods because of the Purkinje shift.
Rubies could also appear more saturate in the late afternoon or early morning, but only if viewed under direct sunlight, the longer (i.e., redder) wavelengths then dominating that light. This is summarized in tables 1 and 2.
Relative color appearance of sunlight and skylight at different times of day
|Sunrise (06:00–08:00)||Very yellowish to red||Very blue|
|Mid-Morning (08:00–11:00)||Less yellowish||Less blue|
|Mid-Day (11:00–13:00)||Least yellowish||Least blue|
|Mid-Afternoon (13:00–16:00)||Less yellowish||Less blue|
|Sunset (16:00–18:00)||Very yellowish to red||Very blue|
Relative color appearance of rubies and blue sapphires under varied natural lighting condtions
|Direct sunlight||Skylight||Direct sunlight||Skylight|
|Most saturate red||Least saturate red||Least saturate blue||Most saturate blue|
|Moderate saturation red||Moderate saturation red||Moderate saturation blue||Moderate saturation blue|
|Least saturate red||Most saturate red||Most saturate blue||Least saturate blue|
|Moderate saturation red||Moderate saturation red||Moderate saturation blue||Moderate saturation blue|
|Most saturate red||Least saturate red||Least saturate blue||Most saturate blue|
Why use north skylight in particular?
It is often heard that north skylight is the best natural light to use for examining gems. As a Singapore diamond dealer recently told the authors: “Diamond dealers from Singapore to Europe who don’t have a GIA Diamondlite handy, examine stones under natural north light. This is common practice.” Many colored-stone dealers also prefer using north skylight.
Besides Wade’s reference to north light, others mention it in the literature. A former employee of the Burma Ruby Mines Company (Keely, 1982) tells the story of a Burmese ruby transaction, in which he relates:
“…The next day, U So arrives at U Pu’s house about 2:45 p.m. and is taken by U Pu to a long room containing a large window facing north. U So takes the proffered ruby in the rough and walks to the window, where he examines it most carefully in the purest northern light which is said to be available at 3 p.m.…”
When discussing the color grading of pearls, another person (Anonymous, 1982) observes:
“As well as the lustre of the pearl the dealer also looks for the colour. This is best assessed when the pearl is placed on a sheet of completely white paper and examined by north light…”
Typical of written accounts and statements such as these, nobody ever explains why north skylight is preferred. What is the reason?
Within latitudes north of the equator, the sun passes over the horizon thru a southerly portion of the sky with respect to an earthly observer. This is especially the case at the higher latitudes, such as New York City (41°N) or Amsterdam (52°22N), where the sun is never directly overhead (at zenith) at any time of the year. Practically, this means that at those latitudes the southern sky contains more In short, if you are looking at stones under skylight and are in San Francisco, New York, London, Amsterdam, Antwerp—or any other major gemstone center well north of the equator—then northern skylight is generally preferred. If you are south of the equator (Brisbane, Santiago, etc.), southern skylight is the general rule. For those actually on the equator (viz., Nairobi), the “best” skylight viewing conditions fluctuate much with the season of the year.
This data is summarized in Table 3. Note that this table serves as a guideline only. For some latitudes at certain seasons (i.e., New York City during the June solstice), the preferred skylight direction literally changes with the time of day. For New York in June, south skylight contains less glare near sunrise and sunset, while north skylight is preferred most of the rest of the day.
Guidelines on when and where to use north versus south skylight
KEY: S = South, N = North, NS = North or South
|If you live in or near|
| Tropic of Capricorn
- Anakie (23°S)
- Rio de Janeiro (22°54S)
- Gaborone (24°45S)
- Tananarive (18°55S)
- Johannesburg (26°12S)
|If you live in or near|
- Bogota (4°36N)
- Colombo (6°56N
- Nairobi (1°17S)
- Singapore (1°N)
|If you live in or near|
| Tropic of Cancer
- Bangkok (13°45N)
- Bombay (18°58N)
- Chiang Mai (18°46N)
- Hong Kong (22°N)
- Jaipur (26°56N)
- Karachi (24°52N)
- Mogok (22°55N)
- Rangoon (16°47N)
- Taipei (25°03N)
|If you live in or near|
| New York City
- Amsterdam (52°22N)
- Antwerp (51°13N)
- Athens (37°58N)
- Beijing (39°55N)
- Idar-Oberstein (49°42N)
- London (51°30N)
- Los Angeles (34°03N)
- Madrid (40°24N)
- Moscow (55°45N)
- Paris (48°52N)
- Peshawar (34°01 N)
- Tokyo (35°42N
Conclusions to Part I, or, why use natural light at all?
Indeed, why use natural light? This is a reasonable question to ask. As we have seen, the color quality of natural light—be it northern skylight or otherwise—is affected by the position of the sun in the sky at a given season (in turn a function of latitude), the path-length of sunlight thru the atmosphere in its relationship to the “scattering” phenomenon, weather conditions and the extent to which dust or manmade pollution permeate the atmosphere. Those are a lot of variables! If one is making critical judgments involving potentially large amounts of money, would it not be better to view gems under one standard type of artificial lighting? This way they would always look the same to everyone, at any latitude, under any weather and/or pollution conditions, at any time of the year.
Yes, but which particular artificial light source would we deem standard? Incandescent? Fluorescent? If the latter, then what kind of fluorescent lighting? A so-called warm white fluorescent light? Cool white? Artificial “daylight?”
For any one individual, it would not matter which of these sources were used, as long as that person used the same one consistently. For most people, though, that is not the practical answer. Gem traders often leave their offices to make purchases (there are exceptions!) and can not be expected to pack around their favorite light source wherever they go. Hence, a universal lighting standard is needed in the gem trade as a whole, so that one gemstone always has the same color appearance to everybody, everywhere, anytime.
But again, which artificial light best qualifies as that standard? And should there be more than one standard: one for dealers and perhaps another for retail outlets where lighting is more often used to exaggerate color appearance? These and other questions are addressed in Part II of this article.
[See Part II in next issue: Artificial Lighting: The Options Available]
- Anonymous (1982) The dealer looks at gemstones. Gems, Vol. 14, No. 6, December, p. 24.
- Ball, V. (1977) Jean-Baptiste Tavernier’s Travels In India, Vol. II. Oriental Books Reprint Corp., New Delhi, pp. 58–59.
- General Electric Co. (n.d.) Engineering bulletin TP-119.
- Keely, H. H. (1982) The ruby mines of Burma. Gems, Vol. 14, No. 5, October, p. 13.
- Mueller, Conrad G. & Rudolph, Mae (1972) Light And Vision. Time-Life Books, New York, pp. 99–102.
- Overheim, R. Daniel & Wagner, David L. (1982) Light And Color. John Wiley & Sons, Inc., New York, pp. 236–240.
- Sersen, William J. (1988) Corundum ‘type’ categories. Gemological Digest, Vol. 2, No. 1 & 2, pp. 3–9.
- Shipman, Carl (1974) Understanding Photography. H. P. Books, Tucson, AZ., p. 157.
- Strahler, Arthur N. (1975) Physical Geography. John Wiley & Sons, Inc., New York, pp. 59–73.
- Varley, Helen (editor) (1980) Color. The Knapp Press, Los Angeles, CA., pp. 162–163.
- Wade, Frank B. (1916) Diamonds: A Study Of The Factors That Govern Their Value. G.P. Putnam’s Sons, New York & London, p. 6.
- White, Harvey E. (1959) Physics: An Exact Science. D. Van Nostrand Co., Inc., New York, pp. 355–357.
- Zucker, Benjamin (1979) How To Buy & Sell Gems. Times Books, New York, p. 17.
1. This comment reminds one of an old method for spotting inclusions in gems using direct sunlight. The stone is cupped in the hand, table up with respect to the eye, and observed with reflected sunlight. This illumination technique can still be observed today in Burma as well as the Chanthaburi and Trat stone markets in Thailand. Cf. V. Ball (1977), wherein the 17th Century traveler, Jean-Baptiste Tavernier, Baron d’Aubonne, says of diamonds in India:
As regards the water of the stones, it is to be remarked that instead of, as in Europe, employing daylight or the examination of stones in the rough, and so carefully judging their water and any flaws which they may contain, the Indians do this at night; and they place in a hole which they excavate in a wall, one foot square, a lamp with a large wick, by the light of which they judge of the water and the cleanness of the stone, as they hold it between their fingers. The water which they term ‘celestial’ is the worst of all, and it is impossible to ascertain whether it is present while the stone is in the rough. But though it may not be apparent on the mill, the never-failing test for correctly ascertaining the water is afforded by taking the stone under a leafy tree, and in the green shadow one can easily detect if it is blue.
2. One of the authors (WJS) once saw a dealer inspecting a blue sapphire at a Bangkok wholesale establishment. While sitting at a long table perpendicular to an east window, the dealer viewed that gem from a seat nearest the window. Suddenly, he moved three seats down from the window (about 2 meters), re-examined the stone, and declared “This sapphire has just dropped $200 in price!”
The point of this story is, of course, that the intensity of natural light can vary tremendously under such viewing conditions. And conditions like these are typical when buying stones at wholesale level in many parts of the world.
You can appreciate this more fully by conducting a simple experiment of your own. Use a hand-held (photographic) light meter to measure light intensity, first from a position immediately next to a window (which may face any direction), then at positions in half-meter increments away from that window. It does not matter if the sky is clear or overcast. In either instance, large drops in recorded light intensity will occur.