Garnet is one of a few gemstones that can exhibit a color change. A color change gem is a stone that changes from one color to another color depending upon the light source. As a general rule, the Madagascar material goes from a raspberry red to a bluish grey. The Tanzanian material goes from a raspberry red to a teal blue color. The red color side of these garnets is typically the strongest, and the blue/green side is the weakest. Until this color change garnet was recently discovered in Madagascar, there were never any blue garnets!
The color change occurs in garnet because of the atomic structure of the stone. The ultraviolet rays in sunlight or fluorescent light excite the atoms in a color change, but artificial (incandescent) light does not. As a general rule when evaluating color change gemstones, the more dramatic the color change, the more desirable the material. You do not want to use your imagination to see the color change. Ideally, you are looking for a 100% color change with two pleasing colors. In reality, you often get a lower percentage color change. You do not want a stone that obviously “bleeds”, which means you can see the two colors at the same time under a single light source.
The more vivid the colors of a color change, the more valuable the gemstone. Ideally, you want a dramatic color change with a medium tone and intense color. As a general guideline with African color changes, you either get a moderate color change with a light tone or a strong color change with a dark tone. However, you must be aware these color changes with dark tones can often black out.