# crystal A crystal facet with real dispersion: red, green and blue read the scene behind it at three different bends Nothing lights this directly, so the whole read is refraction. The scene behind a facet is sampled three times at three slightly different indices of refraction, and the red, green and blue of the result come from three different places. That separation IS the dispersion fringe, and it crawls along an edge because the bend is steepest where the facet turns away. `opacity` is the growth front closing over a face: as it goes to one the facet stops transmitting and turns into black glass rather than a white wall. `emissive_gain` takes the whole set out with it.