A geode is a hollow or partially hollow, more or less spherical tubercle that is covered with crystals on the inside. Usually geodes are between 2.5 and 30 centimeters in size, but this can vary greatly. They are found in sedimentary and volcanic rocks and are sometimes very precious.
A geode consists of a thin but tight outer layer of silicates, usually filled with a layer of quartz crystals such as amethyst or smoky quartz, but may also contain calcite. There are also specimens whose cavity contains water. They would have formed as "bubbles" in the liquid magma that rose in a crater pipe during an eruption. Just as bubbles collect in boiling milk under the membrane that forms on the milk, gas bubbles get stuck in the magma. These gas-filled cavities persist after the magma solidifies and fill with saturated mineral solutions. A geode is formed as a layer of silica in a gas chamber, usually a "fossil" cavity in the rock. The minerals in the water that runs 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 you weigh geodes and compare their weight, the lightest in weight can sometimes contain beautiful crystals. Geodes are sawed in half or broken in half with special pliers to reveal the interior.