Alexandrite is a gemstone coming into popularity because it is one color by day and another color by night. We'll tell you how it manages this color transformation.
Aalexandrite is a saucy mix of aluminum and beryllium with an important dash of chromium mixed in. Properly cut and polished it looks like an emerald or a sapphire.... or possibly a ruby. It all depends on when you look — if you look at it in the daylight it will most likely be an emerald, and at night, it's going to look more like a ruby.
To understand why, let's take a look at rubies and emeralds themselves. Technically, both stones can be many different colors, but generally they're red and green, respectively. Look at them in white light — light that includes every visible color of light — and they are their familiar colors. But try looking at a ruby in light that excludes red wavelengths, and it will be black. If there's no red light going in, none will come out. An emerald can only give off green light if there are green wavelengths coming, and so in light that doesn't include green it also appears black.
Alexandrite can appear as either an emerald or a ruby because it gives off light that is blue-green and orange-red. If it's a really spectacular stone it will appear green in light that contains a lot of blue-green wavelengths, like daylight. In light that contains more red-orange wavelengths, like incandescent lights or candlelight, it will turn red. You may notice that one color is vivid while the other is faint. Since daylight does contain some red light, and incandescent light does contain some blue, both colors are present all the time. One color just overwhelms the other under certain conditions.
Not all stones change color so completely. Many have an overlap between the colors they give off so they look purple in some conditions, blue in others, or green in some conditions and brown in others. But it's the red-green ones that are showstoppers.
[Via Berkeley, JTV]