May 9, 2021
Human Perception and the Color of Gemstones
Originall published in GAMMA The ICA | GemLab Journal — Volume 1 Issue 2
Do you remember a few years back, there was a viral phenomenon going around the internet that involved a dress with a color that no one could agree about? Some people said the dress was blue and black and some said it was white and gold. Over 10 million tweets were made about this photo and it even made its way to the evening TV news. The photo eventually sparked a peer reviewed scientific study about the science of human perception. The manufacturer of the dress eventually confirmed that the dress is black and blue but it’s just one small example that shows how complicated color can be.
The Premise
If you work in the gem trade or if you buy gems, there is a chance that this situation has happened to you: There is a stone for sale in Bangkok, for example. There is a potential customer in the West. For the sake of this example, let’s say they’re in London, though it could easily be Geneva, Paris, New York, or Los Angeles. The seller takes a snapshot of the stone with their camera phone and the customer likes it. Money is exchanged and the seller ships the stone from Bangkok to London. Upon arrival, the customer is shocked to see that the stone is dramatically different from what they thought it would be. They contact the seller and yell and scream and call them a tricky crook. The seller is shocked because they did nothing wrong but somehow the customer feels deceived.
The Breakdown
Let’s look at this chain of events in detail. Color is not a fixed phenomenon. There are a million things that influence how we perceive it and I am going to break them down to (hopefully) make it more understandable.
Sunlight
First of all, there is sunlight. If the seller took the photo outside and the buyer looks at the stone outside, then that is the simplest situation that could happen. Bangkok is in a tropical environment and is close to the equator. The sunlight in Bangkok is very strong and very yellow. London on the other hand is known as a grey city where it rains a lot. In my experience as a photographer, light in the UK is very blue and green. Without any other factors involved, the same stone looked at by the same person, at noon, would look dramatically different in those two different cities.
Time of Day
Now let’s consider the time of day. If the seller took the photo outside at 9am and the buyer looked at the stone outside at 1pm, even in same city, let alone on different sides of the Earth, the stone would look dramatically different. The color of sunlight is not the same all day. It’s warmer in the morning and the evening because the Earth’s atmosphere scatters the shorter light rays (green, blue, purple) and leads us to notice the remaining colors which are warm colored.
Electric Light
So far, the example has been simple. The color of the sun is somewhat of a fixed constant. We know that at noon on most days, the sunlight in Bangkok will be about the same color and we know that the sunlight in London will be a different color. But what if the seller took the photo indoors because it was raining outside in Bangkok. Suddenly the situation gets a lot more complicated because there are many different colors of electric light. The kind of lights that you typically see on the desk of a gem trader are fluorescent. The standard florescent light color is 6000K which is described as “cool white”. This is the same color of light that we see in offices and hospitals. It’s a stimulating color that make the mind more alert. It also makes the color of blue sapphire a little richer and makes the color of ruby look dull.
Maybe our buyer received the stone at home and looked at it at night in the living room. The color of lightbulbs that people use at home are usually around 3500K and are called “warm white”. Warm white looks softer, warmer, and more comfortable to our eyes and also has the effect of enhancing the red color in ruby and making blue sapphire look darker, possibly even black.
We can see that if electric lighting is involved, the situation becomes very complicated. Not only must we consider that the sunlight is a different color between London and Bangkok, but now if the seller took the picture with a cool white bulb and the buyer looks at it with a warm white bulb the stone will appear to be a different color. If the seller took the picture in the warm Bangkok sun and the buyer looks at the stone at work with a cool white light overhead, the stone will also appear to be a different shade of color.
Cameras and Screens
This is where the situation gets infinitely more complex. The seller took their picture on a Samsung phone but the buyer looks at it on an iPhone and then later on an older MacBook Pro. Each cellphone has a certain color profile that it uses. Some photos look better taken with a Samsung, some look better on an iPhone depending on brightness and shade of color. In my experience, I have found that the iPhone does a particularly bad job accurately photographing reds and greens. They both tend to look blown out and dramatically misrepresented and in effect, make the photo look inaccurate.
So, we know that the choice of phone that the seller has used to take the picture with is going to make the stone look different and if the stone is red, which it is, the phone is going to have an especially hard time representing that color accurately. Now the customer looks at the stone photo on a different brand of phone which has a different color profile. It shows colors differently. So even if the seller took a perfectly accurate photo (which is not possible due to all the variables I have already described) the color would look different because the buyer is looking at the photo on a device which makes the color look different. If they buyer would compare the photo on their iPhone XS, which has a Retina Display, to the photo on their older MacBook, which does not have a Retina Display, they would notice, in their own home, in their own lighting, that the color is not the same because the Retina Display has a slightly different color profile than older MacBooks with non-Retina screens.
Color Gamut
On top of color profiles and the various ways that different devices display color there is also something called color gamut to take into consideration. The human eye can differentiate 7 million different colors. When you take an iPhone photo, you are converting all those color variations that your eye can see into an 8-bit JPEG which uses a profile called sRGB. Each device uses a slightly different color range or gamut. When you go from the color gamut of a camera to a computer monitor or a phone screen, the software must convert the colors and this can cause subtle color shifts.
Backlighting
We now know that there is literally no way that the buyer and seller could see the same exact hue in a photo of a red stone. The color of the sun, the color of the electric light, the color profile of the camera, and the color profile of the viewing screen will all affect the hue, saturation, and tone of the stone. But there are still more ways that the color of the stone can change and some of these are dramatic. If you put your electric light in front of a stone it will look one color. If you put your electric light behind the stone (to see the internal inclusions better, for example) the stone can appear to be every different color. If your stone is taken against a white background it can look different than if it was taken with a dark background. This has to do with how dark and light surfaces absorb and reflect color from a light source and also how our eyes perceive color against dark and light backgrounds.
In the end, it’s not possible to avoid this problem. The best thing to do is be as informed as possible about how color works, whether you are the buyer or the seller, and then have a degree of flexibility and an open mind when receiving the stone. If it’s not exactly how you expected it to look, it’s ok to discuss this with the seller and if the stone is not suitable due to differences in color perception then it should be acceptable to return the stone to the seller for a refund. The absolute best thing to do is to see the stone in person in the city that you live in. This isn’t always a realistic option so the best thing we can do is try our best to accurately represent colors and cross our fingers.
Once we own a stone, whether that be in jewelry or in a gem collection, we can watch it all day and night and appreciate all the shades of color that it displays as the sunlight changes color and as we take it from cool light into warm light.
About the Author
Justin K Prim is an American gemcutter. He has studied gemcutting traditions all over the world as well as attending gemology programs at GIA and AIGS. Justin has taught gemology and gemcutting at AIGS and IGT in Bangkok and he has recently published his first book, The Secret Teachings of Gemcutting. He is the founder of Faceting Apprentice, an online gemcutting school, and he also writes articles, produces videos, and gives talks about gem cutting history.
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