"Chalcedony is a cryptocrystalline form of silica," Peridot said, her tone perhaps unsurprisingly excited for someone who was talking about rocks. "It's composed of quartz and moganite. Agate is mostly chalcedony, but it's composed in alternating layers with microgranular quartz!"

She paused, and then added for ease of comprehension, "Agate is striped."