These small hollow geodes are composed of various forms of quartz. The inner core is commonly the crystaline variety, while the outer skin is made up of microcrystaline quartz or agate. Occuring in egg form, these geodes have been sliced in half and polished on the face. Geodes or “ocos”, which are most common in Brazil, are similar to the North American “thunder eggs”.Size, colour and shape vary.