Last weekend I visited my local volcanic park
(every city should have one) and found myself transfixed by these little blobby things: amygdules
. They started out as vesiclesgas bubblesin the basaltic lava that once flowed there some 10 million years ago, but some time afterward, they filled with minerals. As the lava weathered and eroded away, the mineral-filled former vesicles emerged with their obscure new names. You could call them fossil bubbles.
"Amygdule" is a word first found in the 1878 textbook Elements of Geology by Joseph Le Conte. It's a coinage in scientific Latin, from "amyg-" meaning "almond" and "-dule" from "nodule." British geologists tend to favor the spelling "amygdale," which unfortunately can be confused with the medical term referring to almond-shaped glands and structures, like the uvula, or the French word for tonsils.
Amygdules Geology Guide photo