Geode (GK: Earth like)
Geodes are colourful round shaped rocks sometimes found inside of openings of volcanic basalt rock. When the earth erupts and molten lava flow and cools the farther away from its source, gases from down in the earth try to escape to the surface, forming gas bubble openings in the cooler surface rock.
Over a period of time, dissolved silica salt and other minerals such as iron, phosphorous, copper, etc. percolate down from the surface, get super heated and go back up to the surface.
If the conditions for forming geodes are just right, this super heated, highly concentrated dissolved mineral vapour solution will attach and crystallize inside of the original openings formed by the escaping gases of the molten lava rock.
The geode being harder than the basalt lava rock eventually weather out showing the original lava flow pattern on the outside and crystalline agate formation on the inside.