Hard to believe that the two gem materials are simply different shots of the same gemstone. But they are. The difference is due to a phenomenon called color change. In the above photographs you are seeing the same rough piece of synthetic alexandrite that has been photographed in incandescent light and fluorescent light. In incandescent light the gem material appears red. In the FL light it appears green. The reason is somewhat complicated but here are the basics:

Visible light is made up of a combination of colors...like you see in a rainbow. Add them all together and you get white or ambient light. But when you take certain colors away, you will have only one or two dominant colors. Such as when you take all of the crayons away and leave just the yellow and blue. Take these yellow and blue crayons and mark them on the same line...you get green. The same applies to gemstones. Gemstones take most of the crayons away and leave you with only one or two colors, and one will become the final color. This is called selective absorption. As is the case with a red ruby. Rubies take away all of the crayons except dark red and light pink. Put them together and you get medium red. Blue sapphires will be dark blue and green. Put them together in the gemstone and you get a medium blue.

Now, in some rare cases a gemstone will give you two crayons of one color in one type of light, and two crayons of different colors in another type of light. Hence, one color in one type of light, and another in another type of light. And since the incandescent light and fluorescent light are two distinctly different types of light, this is where you see this change of color, as these gemstones offer different crayons in the different types of light.

Now, for all of you FGA students out there, don't think I am being silly talking about crayons rather than formal physics. You are probably reading all the physics stuff right now anyway in your gemmology programme. But for most people, if I started explaining the energy level in electron volts (eV) required to excite the electrons of a chromium (Cr) atom to a higher level of excitation (orbit), and the corresponding energy absorption required for that higher energy level of excitation as related to the visible light spectrum....it would confuse most people, and bore me to tears from trying to keep up with all of it. So work with me here.

Let's just simply stay with the fact that color change gemstones give you two or more crayon colors making one dominant color with incandescent light. And due to the different energy levels, offer you two or more different crayon colors and therefore a different dominant color with fluorescent light. Beyond that, read your FGA course notes very carefully. And call me if you have questions.

If you are not bored yet.....

As you look at the gemstones above you might go one step further to understand how chromium can be the same coloring element for both emeralds (green) and rubies (red). Think again of the crayons and compare the above photographs. The difference is that rubies and emeralds do not give you different crayons in different light. They use chromium to give you two of the same crayons in all visible light. Hence chromium in emeralds is always green. Chromium in rubies is always red. But chromium in the color change gemstone of alexandrite gives you both green and red. If you compare the above photographs I think you will see the connection. If not, call me.