In the southwest corner of the Blue Mountain range in Morrow County, northeastern Oregon, is the site of a large opal mining operation in an area called Opal Butte. The Opal Butte opals occur in rhyolite geodes (also called thundereggs) embedded in decomposed perlite (altered to a pastel clay). These geodes look like regular rocks on the outside but, when broken open, can reveal hidden treasures on the inside! The geodes may contain agate stalactites, banded agate, quartz crystals (rarely), common opal, or various types of gem-quality opal.
Opal is hydrous silica (SiO2·nH2O) made up of extremely tiny spheres (colloids) that can be seen with a scanning electron microscope (SEM).