Geode n. : A nodule of stone, containing a cavity, lined with crystals or mineral matter.

Geode n. : The cavity in such a nodule.

Example:
Can you determine if a Geode contains agate, or banding without cutting it?

Taxonomy
Geodes, cavity, free encyclopedia, rock, Geode State Park, Geode Beds, rock formation, rock cavities, quartz crystals, crystals, sedimentary rocks, Illinois State Geological Survey, search, Carefree Resort, Booklet, scale, earthlike, chalcedony.

Defination from the web:
Geode (disambiguation). This article relies largely or entirely upon a single source. ... Geodes (Greek γεώδης - geōdēs, "earthlike") are geological rock formations which occur in sedimentary and certain volcanic rocks. ...