Lithophysa from France
A lithophysa (plural lithophysae, from Greek lithos "stone" + physan "to blow") is a felsic volcanic rock with a small spherulitic cavity and concentric chambers. Its shape is spherical or lenticular.
These cavities are believed to be caused by expanding gases in tuffs and rhyolitic lavas before solidification. If it has become lined with crystals it may be referred to as a geode or a thunderegg. 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. Said cavities usually contain layers of various colors (red, pink, gray, etc.) composed by crystals of quartz, chalcedony, hematite, fluorite and various colored oxides or other minerals.
Lithophysae are one of the many forms of silica (SiO2), as quartz, agate, opal, chalcedony, etc. They can have an average diameter 5–20 centimetres (2.0–7.9 in), with a maximum of 60–80 centimetres (24–31 in). Lithophysae are also related to the spherulites found in obsidians at Lipari, in Yellowstone Park and other places.
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