Also mistaken for fulgurite by the more naturalistically minded, apparently. Maybe most common in the Nordics, based on viking references?
Additional links:
https://www.nationalgeographic.com/science/article/100810-thor-thors-hammer-viking-graves-thunderstones-science
https://fi.wikipedia.org/wiki/Ukonvaaja [Finnish]
Sidney Harris taught me otherwise.
EDIT: I just saw he turned 91 yesterday. Happy Birthday, Mr. Harris!