Goethite (basically, rust) can take on all sorts of forms. Here, it lines a cavity deep in the interior of a vug, underneath a layer of clear overgrowth that gives this fire agate its internal form! This is rare and precious material, sometimes used by very skilled jewelers. This is so pretty, though, it should remain a mineral specimen but you can see what a beautiful carving can be made to show off these bizarre inclusions. This is an intense and beautiful piece in person! It is so clear and strangely lightweight I mistook this for opal myself, as it was initially labeled here and is sometimes labeled by dealers. But it is technically a form of quartz var. agate, with goethite giving it the iridescence from underneath.