Moonstone and labradorite both do something quartz and calcite never do: hold perfectly still and let a soft light appear to move across the surface as you tilt them. It looks like the stone is glowing from somewhere underneath. It’s neither glow nor pigment, it’s light bouncing off internal structure that’s too fine to see directly.
Two feldspars, two related effects
Both minerals are feldspars that cooled slowly enough for two chemically similar feldspar phases to separate out into ultra-thin, alternating internal layers, a process called exsolution, essentially the mineral unmixing into microscopic stripes as it cooled below the temperature where the two phases could remain blended. In moonstone, light reflecting between those thin layers produces adularescence, a soft, billowing blue-white sheen that seems to float below the surface. In labradorite, thicker and more regularly spaced layers produce labradorescence, a bolder flash of blue, green, or gold that appears and disappears sharply as the viewing angle changes.
Structural color, not pigment
Neither effect comes from any dye, coating, or trace-element color center the way amethyst or smoky quartz’s coloring does. It’s a purely optical phenomenon, thin-film interference, the same physics behind the color you see on a soap bubble or an oil slick, just happening at a microscopic scale inside solid feldspar instead of a fluid film. Because the color depends entirely on layer thickness and spacing rather than chemistry, two labradorite specimens from the same locality can flash completely different colors depending on exactly how their internal layers formed.
It’s a related but distinct mechanism from the fibrous-inclusion trick behind chatoyancy and asterism, both are optical effects that make a mineral appear to move, but one comes from layered interference and the other from light reflecting off parallel needle-like inclusions instead.
