Geode pendant with Citrine, Amethyst or Rock crystal point from Brazil

A geode or druse is a hollow or partially hollow, globe-shaped tuber that is covered with crystals on the inside. Typically geodes are between 1 and 12 inches in size, but this can vary widely. They are found in sedimentary rocks and are sometimes very expensive. 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 it can also contain rutile or calcite. There are also examples of which the cavity contains water. They are said to have formed as "bubbles" in the liquid magma that rose up a crater pipe during an eruption. Just as bubbles collect in boiling milk underneath the skin that forms on the milk, gas bubbles get stuck in the magma. These gas-filled cavities persist after solidification of the magma and fill with saturated mineral solutions. A geode is created as a layer of silica in a guest chamber, usually a fossil cavity in the rock. The minerals in the water, which 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 weight can sometimes contain beautiful crystals. Geodes are sawn through or broken in half with special pliers to reveal the interior.