Geodes are the mysterious treasure-boxes of the geological world. Undistinguished plain rock from the outside, they often reveal crystal-lined interiors when cut or broken open. The crystals are most often clear quartz, although they are sometimes amethyst or calcite. Rarely, crystals of pyrite, sphalerite, and other minerals may also be found. Geodes may be less than an inch in diameter, though some, like the Brazilian amethyst cathedrals, can be several feet across.
Some geodes, commonly referred to as duds, are empty. Others are solid crystal, or nearly so; these are called nodules. There's no way of telling what you will find in a particular geode from looking at the outside, although nodules are noticeably heavier than hollow geodes.
Geologists don't agree on the exact processes involved in the formation of geodes. Given that geodes form in both volcanic and sedimentary rocks under very different conditions, the subject is a complex one. But the most common theory is that geodes form inside already existing hollows within the rock. In the case of volcanic rock these hollows are the result of gas bubbles in the molten flow. Cavities in sedimentary rock may be the result of concretions, of an expansion in the rock due to internal fluid pressure, or of the dissolving out of earlier material by groundwater – or any combination of these causes.
Groundwater ladened with silica and other minerals fills these hollows. Over thousands of years minerals precipitate out of the water, leaving a silica gel on the interior walls of the cavity that hardens into rock as it dries. The first layer is usually chalcedony, a strong, crypto-crystalline form of quartz. As this process of mineral precipitation reoccurs over and over, later layers form distinct, inwardly pointing crystals. Geodes that are empty missed these later cycles. When a number of geodes are found together in a layer of rock, often it's the ones at the top – ones that were often above the level of the groundwater -- that are duds.
A similar process of mineral precipitation can create crystal-lined cavities called vugs. The difference between one of these cavities and a geode is that the outer layer of a vug is not durable enough to survive weathering, so it disintegrates when exposed rather than forming a ball of rock with a crystalline mystery at its heart.