Let’s do a legit Thin Section Thursday post. This is a metamorphic rock viewed beneath a microscope with a polarizer below the slide and a polarizer above the slide. The lower polarizer blocks light except light polarized in one direction, then when it hits a crystal that light changes direction and is partially delayed, creating colors the more it is delayed. The light goes through the second polarizer and any light that isn’t re-bent will be blocked out. The dark areas in this clip either don’t let light through or don’t bend it, so the upper polarizer knocks it out. The rainbow pattern is mica minerals - they often grow in horizontal sheets and define flat planes in metamorphic rocks. Those mica minerals are defining a foliation in this rock.
