Pleochroic halos (also referred to as radiohalos) are microscopic, spherical shells of discolouration (pleochroism) within minerals such as biotite that occur in granite and other igneous rocks. The shells are zones of radiation damage caused by the inclusion of minute radioactive crystals within the host crystal structure. Joo's ongoing interest in geological timescales persist in this series, but this particular topic also intersects with a human cosmological schema: Radiohalos remain mysterious even to modern chemists and geologists, and have been cited by Creationists for many decades as evidence of a young Earth. While scientists have put forward speculation towards possible origins of the halos, the ambiguity surrounding them has kept them open as a symbolic locus for meditation on divine structure of the universe.