Agate is classified as an igneous rock, a rock that is created from molten minerals, elements or rocks.  Inside the bottom of the earth's crust near the surface of the earth's mantle the rocks are molten.  Inside the molton rock gas bubbles form.  When seismic forces push the the molton rock away from the intense heat, the rock turns solid leaving empty gas bubbles. Over thousands of years, ground water from the earth above flows into these pockets. The water contains various minerals.   When the water evaporates over a long period of time, the minerals precipitate out of the water and form each layer of the agate.  Agate is microcrystalline and many times pure crystals form inside the the final stages of the agate formation.

There are so many elements and minerals involved in the formation of a geode. Since the earth's crust is approximately 72% silicon dioxide (SiO2) , it is the main content of an agate geode. Some geodes still contain some of the water that formed them. The age of a geode has never been proven as they form at different rates according to the conditions in which they grow.