Description
A geode or druse is a hollow or partially hollow, globe-shaped nodule lined with crystals on the inside. Geodes are usually between 2.5 and 30 centimeters in size, but this can vary greatly. They are found in sedimentary rocks and are sometimes very valuable. A geode consists of a thin but solid outer layer of silicates, usually filled with a layer of quartz crystals such as amethyst or smoky quartz, but it may also contain rutile or calcite. There are also specimens whose cavity contains water. They are believed to have formed as “bubbles” in the liquid magma rising in a crater pipe during an eruption. Just as bubbles collect beneath the film forming on boiling milk, gas bubbles get trapped in the magma. These gas-filled cavities persist after the magma solidifies and fill with saturated mineral solutions. A geode forms as a layer of silica in a gas chamber, usually a fossil cavity in the rock. The minerals in the water running along the hardened outer shell form crystals in the silica layer. Crystals then slowly grow in the cavities. Agate is formed in the same way. If one weighs geodes and compares their weights, the lightest one can sometimes contain beautiful crystals. Geodes are sawn in half or broken with special pliers to expose the interior.
