Did you know there are such things as color changing gemstones? Some exquisite gemstones change color under different lighting sources from natural sunlight to florescent or different internal lights. These color changing gemstones come from many different families of minerals including Alexandrite, Garnets and some Sapphires.
There is still a lot of confusion regarding color changing gemstones. Some people are still using the old technique of checking between incandescent and fluorescent lights, which is inadequate. Worse still, some people aren’t checking for color change at all. This is one of the most significant weaknesses of modern gemology and one that needs to be addressed regularly.
The most expensive and well known color changing gemstone is the Alexandrite. This gemstone changes from traffic light Red to traffic light Green. It really is incredible.
First let me point out that color change is defined by what you see between natural and artificial light. If you think about it for a moment, it will be obvious. Electric lights have only been around for a bit over a century, but the color change phenomena in color changing gemstones has been know for much longer. Originally, it was defined as the difference people saw between sun light and fire light, be that from a candle, oil lamp, or whatever. As incandescent light became common, the effect was the same as comparing sun light with fire light. so the definition was modified to be a comparison between natural and electric lights.
Below are pictures of a Garnet. This is one of the most widely available color changing gemstones on the market today.
This is the standard for defining color change in color changing gemstones: it is the difference between what you see in natural, (I.E. sunlight,) and any other light source.
The definition of color changing gemstones needed to be modified again when fluorescent lights were added to the equation. They often show the same hues as natural light, which is why people began checking for color change with two types of electric lights. However, we now know of color changing gemstones that do not react to fluorescent the same as natural light. So it is no longer an adequate to use two electric lights when checking for color change.
Many people only check a few gems for color change. That this is unfortunate can be shown by one glaring example. Some years ago, one of Hollywood’s most prominent families sent some diamonds to the GIA for grading. The reports came back with no mention of them being color changing gemstones, something that greatly effected their value. For security reasons, there were no windows in the grading area. And, since no one had ever heard of color change diamonds before, the test was not considered necessary. We now know that color changes Diamonds are extremely rare and valuable. They are commonly referred to as Chameleon Diamonds.
Below are pictures of color change Andesine. This is another mineral where color change was not given much thought. Every gem should be checked for this property.
In the last couple decades, many new color changing gemstones have emerged and new ones are continuously being discovered. On Gem Rock Auctions there are color change Sphenes being offered for sale. I have never heard of a color change Sphene before, but obviously they do exist. If someone hadn’t bothered to check these gems, they would have been overlooked.
The lesson is simple; you need to check all gems for color change. We do not know all nature has to offer us and there is no excuse for missing such an important piece of information.
The actual mechanism behind color changing gemstones is quiet simple. Every light source has a different fingerprint. That means that visible lght can be made up of many different wavelengths that show different colors.
In the image above it is easy to see that noon sunlight (the yellow line) has a relatively flat line that contains almost every color. Now if you look at the Fluorescent lamp you will notice there is not much blue but lots of yellow and red.
Now lets take Alexandrite. It is Green in daylight and Red in incandescent light. Looking at the yellow and blue line above it is easy to see that daylight has more green color than red color. So to the human eye, the Alexandrite will look green. Now if we look at the blue line you can see the green is relatively low compared to the red. So to the human eye the Alexandrite will look red.
This is a very simplified version of what actually happens. The more complex explanation will involve talking about selective absorption of wavelengths. That is a conversation for another day.
The process is becoming still more complicated. Recently there was a discovery of two gems that showed a three way color change; with one hue showing in natural light, another under incandescent and still a third in fluorescent.
With so many variations now appearing, it is possible there are gems that only show their color change between natural and fluorescent lights, but none between natural and incandescent. I have never heard of such a stone, but the possibility exists.
There is a new phenomena called the Usambara Effect which also creates a color change. However, it is based on an entirely different principal than previously known. This effect an be shown in more than just gemstones. Pumkinseed oil is a fantastic example of this effect. In the bottle this liquid looks red, but when it is poured into a spoon this is what happens:
Around the edges of the liquid it turns green. Why does this happen? Put simply this is caused by optical absorption. it is directly related to the light transmission path. Now in English - the longer the light has to travel through the liquid, the more a certain wavelength will be absorbed. So in the spoon above, the light does not have to travel through the liquid very far and so the green wavelength is able to make it through the liquid as it travel through it.
Now when the liquid reaches a certain thickness, the green wavelength of light is absorbed so much that it stops being transmitted. The only wavelength left is the red.
This effect has been found in Tourmalines. Below ia picture of a Tourmaline showing a green outer edge and red inner edge.
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