Geode interior Quartz crystals centre inside a geode A geode forms when layers of the mineral silica silicon dioxide precipitate from liquid onto the inside of hollows in rock The silica has an outer layer of chalcedony amorphous or non crystalline silica with the layers becoming more crystalline towards the centre Quartz is the crystalline form of silica This geode has been cut open to reveal this crystalline interior A gradual gradation from the outer chalcedony layers red to the inside is seen The colour is due to impurities in the silica