Mushroom Jasper ("Pilzjaspis") of the Dead Horse Wash in Maricopa County, is a strongly silicified rhyolite with significant spherulitical structures. These Spherulites are sometimes quite large and show agate formation inside and / or macro-crystalline quartz. In fact these Spherulites, are Lithophysae ("Thundereggs") in a solid combination with their hostrock, the rhyolite. Similar material from Australia is called "Rainforest Jasper".