A geode is a beautiful mineral formation that usually consists of an exterior shell of chalcedony (very fine-grained quartz), with other minerals and / or crystals lining the interior.
The name "geode" is derived from the Greek word "geo" that refers to the shape of the earth, since most geodes are spherical.
A geode can form in any cavity within rock. Typically, bubbles of gas form in cooling ash beds. When such a gas bubble gets “frozen” in a particular location when the silica-rich ash/dust hardens the process of geode-formation begins. The partial or complete filling of the bubble with the minerals that may lead to the crystaline structures that make geodes so attractive may occur soon afterwards - or millions of years later - and can include a wide range of different minerals.
Geodes are sometimes known as "thunder eggs".
See also the main IvyRose Holistic section about Crystals & Gemstones.