text: I agree with them , mostly u r taliking about geodes !!! \n\nGeodes (Greek geoides, "earthlike") are geological rock formations which occur in sedimentary and certain volcanic rocks. Geodes are essentially rock cavities or vugs with internal crystal formations or concentric banding. The exterior of the most common geodes is generally limestone or a related rock, while the interior contains quartz crystals and/or chalcedony deposits. Other geodes are completely filled with crystal, being solid all the way though. These types of geodes are called nodules.\n\nGeodes can form in any cavity that is buried. These can be gas bubbles in igneous rocks, pockets under tree roots, vesicles in lava after a volcanic eruption, or even animal burrows. Over time, the external wall of the cavity hardens, and dissolved silicates and/or carbonates are deposited on the inside surface. Over time, this slow feed of mineral constituents from groundwater or hydrothermal solutions allows crystals to form inside the hollow chamber. Then, over millions of years, the geode makes its way back to the surface through normal geologic processes.\n\n4 more info and photos try \n\nhttp://en.wikipedia.org/wiki/Geode\n\nhttp://images.search.yahoo.com/search/images?p=Geode&fr=yfp-t-501&toggle=1&cop=mss&ei=UTF-8