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One of the most exciting and interesting gemstones you can buy is one that has color change, Alexandrite is most recognised of these gemstones as it has a dramatic color change depending on your light source. Today there are a lot of color shift gemstones, being sold as color change. Usually a color change gemstone commands a premium price, so how can you be sure you are getting color change and not color shift?
All of a gemstones color (hue, tone and saturation) is dependent on two things, how the gemstone is cut and the most important component light. It is the type of light and direction of the light that produces the brilliance and fire in a well-cut diamond or the beautiful color in gemstones.
The color you see in a gemstone is the result of the light that returns to your eyes after bouncing off the gemstones crystal structure. Now we know what causes color but what makes it a color change or color shift gemstone?
The element chromium that allows equal amounts of red and green light to pass through it causes color change in Alexandrite. Because of this balance of red and green, if you view Alexandrite under incandescent light (rich in red) the gemstone looks like a red Ruby. If you then take the gemstone and hold it under an incandescent light (rich in blue/green) the Alexandrite looks more like an Emerald Green. This is color change.
Color shift is representative of colors that can change in the different or the same light source, a good example of this would be Tanzanite that will shift from blue to violet/purple by tilting the gemstone in the same light source. You will find a reasonable amount of gemstones will have this effect especially the ones with rich colors,
The best explanation of the difference I have used for deciding what is color change and what is color shift, is for those that have seen or have access to color grading wheel which is a chart that designates all of the colors of gemstones for grading gemstones. GIA calls it the hue wheel and it shows all of the color hues from Blue, to Green, to Yellow, to Orange, to Red, to Purple, to Violet (32 hues in all). If a color change moves from Red to the next color on the wheel (orange) then this is called a Color Shift. If the color moves from Red to Green, which is on the other side of the wheel, then this is called Color Change.
Some of the major natural Color Change gemstones are Sapphire, Ruby, Alexandrite, Spinel, and Garnet. Also be aware, today there are a lot of coated or diffused gemstones that will also show color change.
