Description
A geode, also called a druse, is a hollow or partially hollow, spherical nodule lined with crystals. Geodes typically range in size from 2.5 to 30 centimeters (1 to 12 inches), but there are exceptions. They are found in sedimentary rocks and can sometimes be very valuable. A geode has a thin but firm outer layer of silicates, often filled with a layer of quartz crystals such as amethyst or smoky quartz, although rutile or calcite can also be found. Some geodes even contain water within their cavity. They are thought to have formed as "bubbles" within the liquid magma that rose up from a vent pipe during a volcanic eruption. Just as bubbles collect under the skin that forms on boiling milk, gas bubbles remain trapped within the magma. These gas-filled cavities persist after the magma solidifies and fill with saturated mineral solutions. Geodes form when a layer of silica forms within a host chamber, usually a fossilized cavity within the rock. The minerals in the water flowing over the hardened outer shell form crystals on the silica layer. Crystals then gradually grow in these cavities. Agate is formed in a similar way. When geodes are weighed and their weights compared, the lightest one can sometimes contain the most beautiful crystals. To reveal the interior, geodes are sawn in half or broken with special pliers.
