The Contact: Cross Polarized Granite (2015): My favorite detail of the whole show. This is a pleochroic halo (black) around a zircon crystal (blue). Zircon crystals often contain quite a bit of uranium. Particles produced from radioactive decay of the uranium damage the crystal structure of whatever is around the zircon crystal. This causes the optical properties of the damaged zone to change, often making a halo around the zircon crystal change colors as you turn it in polarized light. This play of colors is called pleochroism. It makes it easy to find zircon and other radioactive minerals, which can then be used to date the rocks they come from.