Yes, it’s a thunderegg! Arizona Mushroom Rhyolite, sometimes known as a Jasper. These are often highly free-form masses and used for carvings and cabochons, but this is a nicely thunderegg shaped specimen. I am told it is from ‘Bed 1’ of the location. It appears (almost) completely agateless – a quartz ball. But the charm of this stone lies in the rhyolite matrix and the gorgeous red and black sketched patterns. Definitely a keeper!