Yep, that's a thunder egg, with red chalcedony in the centre. Very pretty rock. Will polish up nice too
The rock itself is a rhyolite porphyry...those crystals you see in the rock are mostly plagioclase feldspar laths. You do rarely get laths of quartz, but acidic rocks like rhyolite normally don't form them because the silica content of the rock is too high (>69%)...they can't nucleate out of solution. Those metallic looking ones are biotite mica crystals (phenocryst is the technical term for crystals that grow large in the mineral groundmass of the rock). The chalcedony forms when silica rich groundwaters penetrate cavities in the rock and evaporates, leaving behind the silica. The silica itself forms a water rich cryptocrystalline to amorphous mass in the cavity. It's a very similar process to how opal forms and how gold nuggets form
It's not metamorphosed in the strictest sense....acid volcanics like rhyolite metamorphose to quartzite and quartz rich gneisses when put under heat and pressure. It may have undergone a form of metamorphism called diagenesis, but by the look of the rock I don't think so. The conditions which are typical of diagenesis normally won't affect volcanic or plutonic rocks, unless there's something atypical of the conditions. They usually just weather out and turn to soils in these conditions.
