Geode Small Quarter to Half-Dollar Size Cut and Polished w Info Card...Millions of years ago when the Earth's crust was being formed, gas pockets and cavities formed in relatively low temperature, volatile magma. Later, after the magma hardened into rock, high temperature fluids filled these gas pockets, depositing minerals and causing crystals to form unobstructed during their growth.      These geodes are made of agate and the crystals in the center are quartz. They have been cut in half and polished on the cut side.