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thunder egg

American  

noun

Geology.

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

Etymology

Origin of thunder egg

First recorded in 1940–45

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.