This rock isn't being lit from behind and it's not see-through. It, like others, seems to light up internally. This internal glow baffled people for centuries, until a combination of physics and geology gave us a clue as to cause of the Schiller Effect.


The Stones That Glow From the Inside

The stone above is labradorite. It is displaying a rather unromantically-named quality called labradorescence. It's not quite iridescence; iridescence seems to originate on the surface of an object. Labradorescence comes from inside the object, lighting it up internally. If you turn a piece of labradorite it will flash like lights are being turned off and on inside of it. Labradorescence is joined by the more melodic adularescence, which is named for adularia, or moonstone. Unlike labradorite's irregular flashes, a moonstone has a gentler and more regular glow.


Both of the stones, as well as a few others, display a quality that people call "schiller" or "shiller." The Schiller effect is seen in a few different stones, including rare samples of such common stones as quartz. It's rooted in both geology and physics.

How To Make a Moonstone

A moonstone is just specially-made feldspar, which is common as mud. In fact, as they are a group of minerals that make up around 60% of the Earth's crust, they are probably a lot more common than mud. They come in many forms, but all those forms include sodium, potassium, and aluminum in different compositions. At one extreme end of possible compositions is feldspar with a lot of sodium, and at the other is feldspar with a lot of potassium. If feldspar material is melted down and begins to reform, the sodium becomes solid first. If there's nothing but sodium-rich material around, this does nothing strange.


If the melted mix is a bit more heterogeneous, things get interesting. On the first edge of the stone to cool, the sodium rich material, called albite, solidifies. The potassium rich material, called orthoclase, solidifies next. This process sucks up all the potassium in the nearby areas, so what's left is more sodium rich material, which solidifies, and takes up the nearby sodium. This leaves more potassium rich material, and the layers just keep coming. Although labradorite is a different kind of feldspar (and is often much darker material) it follows the same process.

The Layering Makes the Stone Shine

How does layering make the stone light up? Think of the simple case of soap bubbles. When light hits the surface of a soap bubble, some of the light reflects back. The rest of the light travels on through the film of soap, but when that light transitions to the air inside the bubble, some light is again reflected back. That's two waves of light being reflected outward towards the onlooker. Because the light waves are slightly out of sync with each other, the peaks and troughs of the two waves overlap to make different frequencies of light, which we see as different colors. As the angle of light changes, and the thickness of the soap bubble changes, the frequencies of light change and we see swirling patterns of color.


The same thing happens inside our various configurations of feldspar. The light reflects off the layers of slightly- different feldspar, and the out-of-sync waves overlap and interfere with each other to make different colors. The difference is the layers don't change thickness the way they do in bubbles, only the lighting changes. In labradorite, the layers are different thicknesses in different areas, so they reflect back blue, pink, green, or yellow. They are also frequently broken up, so we only see them in flashes. Moonstone has more regular layers, and so reflects back a regular blue glow everywhere. (For the most part, moonstones shine blue, but rare varieties shine yellow or orange.)


And Now Some Chatoyance

In the moonstone above, you'll notice there is a strong blue streak instead of a diffuse glow. This is an example of another property, called chatoyance or chatoyancy. The most famous chatoyant stone is tiger's eye, but other stones can be chatoyant if they're cut right, and if they have the right internal structure. Tubes, fibers, or layers line up in a stone to create this effect, and the streak of light coming off the stone is always perpendicular to the fibers.


The best way to picture the physics of it is imagining the streak of light off an old record, always perpendicular to the grooves in the vinyl. Another good way to understand the effect is to picture the glitter path that appears on the ocean when light from the moon hits it. The path of the light is always perpendicular to the waves, as their angle to the incoming light reflects it back towards the onlooker.


Only certain moonstones have that property, and the glitter is muted by the diffusion of light within the stone, so it looks like an ethereal, almost star-like streak of brilliance. So if you want a good look-alike for the phial of Galadriel, search for a chatoyant moonstone.

Image: UCL Mathematical and Physical Sciences c/o:Mary Hinkley.

[Via Atmospheric Optics, The Lunar Glitter Path, Mineral Optics, Adularescence]