Geodes need lots of time and just the right amount of pressure, chemicals, fluids in order to form. They also need something to form the cavity. This could be the body of a dead sea animal, a bubble in volcanic rock or any type of matter that falls to the ocean floor. Over time the dead sea animal or matter becomes buried under layers and layers of sedimentary rock. Pressure compresses the layers and chemical reactions turn them into stone.
Salt water is trapped and eventually seeps through the cavity.
Over thousands of years, silica in the water forms a thin wall on the inner surface. Chemical changes in the water over thousands of years allow the cavity to expand. Millions of years later, the earth s crust moves and the ocean floor becomes dry land. Less salty water can now enter the cavity and allow quartz ,calcite and numerous other minerals to form on the inside. Geodes that are completely filled with crystal and mineral formations are called nodules.