Read an Academic Passage

The Formation of Geological Geodes

Geodes are spherical or subspherical rock structures with an internal cavity lined with mineral crystals. They are often unassuming on the outside, appearing as ordinary rocks, but their interiors hold a sparkling, crystalline beauty. The formation of a geode is a long, two-stage process that can take millions of years. It begins with the formation of a hollow bubble inside a layer of rock. In volcanic rock, these bubbles are often created by gas trapped in cooling lava. In sedimentary rock, hollows can form from the decay of organic material like a tree root or an animal burrow.

Once the cavity is formed, the second stage begins: the deposition of minerals. Groundwater or hydrothermal solutions rich in dissolved minerals, such as silica or calcite, seep into the hollow space. As conditions change, or as the water slowly evaporates, these minerals begin to precipitate out of the solution and form crystals on the inner walls of the cavity. The process is extremely slow, with layers of crystals gradually building up over immense periods. The specific type of mineral and the conditions during formation determine the color and size of the crystals inside.

The variety of crystals found within geodes is vast. The most common are quartz and its varieties, like amethyst (purple) and citrine (yellow), as well as calcite and celestite. The outermost layer of a geode is typically composed of a more durable mineral, like chalcedony, which protects the delicate crystals inside. Geodes are found in specific geological regions around the world, often in deserts or areas with significant volcanic history, such as in Brazil, Uruguay, and Mexico.