The Johnstown meteorite fell the afternoon of July 6, 1924, in Weld County, Colorado. It is a diogenite, part of the HED (Howardites, Eucrites and Diogenites) group of achondrites, which originate in the crust of Asteroid 4-Vesta. According to radioisotope dating, the HED achondrites crystallized between 4.43 and 4.55 billion years ago.
Diogenites are plutonic igneous rocks that form deep in the crust and cool very slowly, resulting in large crystals (note the green orthopyroxene crystals visible in Johnstown).
Diogenites are named for the ancient Greek philosopher Diogenes, who was the first to suggest that meteorites were not terrestrial rocks and that they actually originated in space.
Photo © ASU/CMS