The geod
The geod (geode (from the Greek, γεοειδής or γεώδης - "earthlike") denotes the object, a form similar to the Earth, in this case refers to the spheroidal (rounded) formation in the ground.The bark of such formation is hard, inside - a cavity, sometimes completely filled with crystallized minerals, druses of a semi-precious stone, sometimes free in the center.
Geodes are formed by the natural growth of crystals inside a closed cavity in the rock. Mineral substances, deposited within the ghost, do not necessarily crystallize. The most attractive are the secretions with the internal layers of the semi-precious stone.
The usual size of the ghost varies from one centimeter to one meter in diameter. Small (less than 1 cm) ghosts are called tonsils.
Large geodes - especially large enough for a person to enter them - are commonly called caves and usually have personal names. Academic science tends to call any (regardless of shape and size) ghosts cavities in rock massifs "overgrown" with crystals of natural minerals from within.
photo: Tucson Gem & Mineral Show