thunder egg

noun

, Geology.

1. a globular concretion of opal, agate, or chalcedony weathered out of tuff or basalt.

Word History and Origins

Origin of thunder egg¹

First recorded in 1940–45

Example Sentences

Steve Manford, who works at clothes shop Thunder Egg, said the arcade had had "a renaissance" in recent times.

From BBC

“It was like a thunder egg,” he says.

From Seattle Times