What is a lithophyse? A thunderegg? A Porhyrkugel?
These are names for one and the same thing; more or less round shaped stones occurring in volcanic areas. Lithophysa (Greek) means ‘stone cavity’, a stone with a cavity, it is the scientific name of the subject.
The concept ‘thunderegg’ is a regional name and one version of its origin is that it stems from a legend of the North American Warm Springs Indians. The legend tells how mountain spirits, living at Mount Hood and Mount Jefferson, were fighting each other. They pelted each other with eggs which were stolen from Thunderbirds: the thundereggs which now can be found at the foot of the mountains.
Porphyrkugel and Rhyolithkugel are the names used in Germany and are concerning the geological deposits (Porphyr, Rhyolite) in which they are found; Kugel means rounded stone. Schneekopfkugel is a name from the Thuringian Forest, named after the mountain Schneekopf where nearby lithophyses are found. Also Sternachat is regularly used and refers to the often star-shaped cavity filled with agate.
Nowadays the name thunderegg increasingly is used worldwide, partly as a result of (English-language) internet sites and social media, as the most common term.
Not everywhere where volcanoes were active, lithophyses can be found. The forming of a thunderegg is linked to a number of conditions. The forming of a thunderegg.