What is a Geode?
A geode is a rock containing a crystal-lined cavity or another type of mineral matter; when a geode is opened, you'll see agate, quartz, amethyst or other minerals. The exterior of a geode looks bumpy and might resemble the head of cauliflower. A nodule is a geode that is full of minerals, but it's not hollow. They fill display cases and museums everywhere.
Where do geodes come from?
A geode begins when a cavity forms in a rock, which can happen several ways. A cavity is usually formed in igneous rock created by cooling lava or magma; a bubble of carbon dioxide and water vapor forms within the lava, as the molten rock, cools and the gas dissolves, space is left behind. Another way a cavity can form is when lava solidifies underwater. The outside of the melt solidifies before the inside does, once the liquid has leaked out, an empty pocket of lava is left.
Geodes can also form in sedimentary rocks (rock that has formed through the deposition and solidification of sediment). The cavity in this rock is usually formed from a solid core, a mass of minerals, organic material or nodule, in the sediment, which begins to dissolve leaving a space behind. The result of this leaves a hollow space formation behind waiting to be filled.
Next, for both igneous and sedimentary geodes, mineral-rich groundwater or rainwater seeps into the empty pockets and fills the empty cavity. This water lines a thin crust of minerals inside of the cavity. As the water flows through the geode, additional mineral layers are deposited in its hollow interior.
Over thousands of years, the minerals build crystals that eventually fill the cavity. The type of mineral that ends up in a geode varies by the location, the condition, and type of rock the geode forms from.