Why Do some Gemstones colour change?
Some exquisite gemstones change colour under different lighting sources from natural sunlight to florescent or different internal lights .
This Optical effect gives stunning display hues of different colour. Gemstones include Alexandrite,Garnets,and some sapphires.
Specimens for your viewing pleasure
COLOUR CHANGE ANDESINE COLOUR CHANGE GARNET COLOUR CHANGE ANDESINE
COLOUR CHANGE GARNET COLOUR CHANGE ALEXANDRITE COLOUR CHANGE GARNET
COLOUR CHANGE ALEXANDRITE COLOUR CHANGE GARNET FROM TANZANIA
COLOUR CHANGE GARNETS
There is still a lot of confusion regarding color change gems. 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.
Defining Color Change
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 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.
This is the standard for defining color change: it is the difference between what you see in natural, (I.E. sunlight,) and any other light source.
The definition of color change needed to be modified again when fluorescent light 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 gems 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.
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 change stones, 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.
In the last couple decades, many new color change gems have emerged and new ones are continuously being discovered. In today's classifieds there are two 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 process is becoming still more complicated. I recently discovered 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.
In our lesson on Determinative Gemology I describe an easy technique of checking all gems with both natural and incandescent light. That may well be inadequate. 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. I just this week got my hands on one to examine and I hope to have a full report for you soon.
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