“Fire agate may be one stone, but it is two minerals: chalcedony agate and iron oxide, usually in the form of goethite or limonite. Chalcedony was deposited little by little in bubbly botryoidal layers. Every now and again, a hair-thin layer of iron oxide formed, only to be covered by more chalcedony, and those ultra-thin metallic layers give fire agate its fire thanks to the Schiller effect.”
Photo
by
Jim
Brace-Thompson