A lithophysa is a spherulitic bubble or cavity found in felsic volcanic rocks and glassy basalts and certain rhyolites. It is believed to be caused by expanding gases in tuffs before solidification. If it has become lined with crystals it may be referred to as a geode or a thunderegg. The word lithophysa is from the Latin for rock bubble. The term vug is also used for similar cavities although vug is usually restricted to mean cavities in rocks formed by the removal of material such as soluble minerals.