Alexandrite, the birthstone of June, is an interesting gemstone as it appears to change color when seen under different light sources. For example, in the sunlight it has a bluish-green color, but when seen under incandescent light it has more of a reddish tone. Alexandrite is one of three types of chysoberyl, a gem made up of beryllium.
This stone is particularly rare as it is not often produced in nature because the conditions need to be just right. It is formed with chromium atoms replacing aluminum atoms which is what allows the stone to have its characteristic color changing properties. The more drastic the color change, the more valuable the stone is. Because of this, it is rare to find an alexandrite larger than 3 carats.
The most rare alexandrite gemstone is one with a drastic color change under different types of light, with more common stones having a minor change.
