Dugway geodes from Atah U.S.A. Large format 65-90 mm

A geode, also called druse, is a hollow or partially hollow, spherical tuber that is lined with crystals on the inside. Geodes usually range in size from 1 to 12 inches, but there are exceptions. They occur in sedimentary rocks and can sometimes be very valuable. A geode has a thin but sturdy outer layer of silicates, often filled with a layer of quartz crystals such as amethyst or smoky quartz, although it may also contain rutile or calcite. Some geodes even contain water in their cavity. They are thought to have formed as 'bubbles' in the liquid magma that rose up a crater pipe during a volcanic eruption. Just as bubbles collect under the sheet that forms on boiling milk, gas bubbles become trapped in the magma. These gas-filled cavities persist after the solidification of the magma and fill with saturated mineral solutions. Geodes form when a layer of silica forms in a host chamber, usually a fossil cavity in the rock. The minerals in the water that flows past the hardened outer shell form crystals on the silica layer. Crystals then gradually grow in these cavities. Agate is formed in a similar way. If one weighs geodes and compares their weight, the lightest one can sometimes contain the most beautiful crystals. To reveal the inside, geodes are sawn in half or broken with special pliers.