Geodes are the discrete bodies of mineral matter, commonly spherical, hollow, and lined inside with crystals of various minerals. Geodes necessarily originate from cavities in rock. Deposition of minerals proceeds inward from the cavity wall, producing a characteristic drusy structure, so that the youngest mineral generation is near the center and may fill the void completely. In a geode, the mineral layers lining the cavity must be more resistant to weathering and erosion than the host rock, so that upon weathering, a discrete, hollow mineral body is released.
Any cavity in rock is potentially the site of geode formation. In a particular geode occurrence, the key question is the origin of the cavity. Geodes have formed in vesicles of lava flows, such as those of the Columbia Plateau. Miarolitic cavities in plutonic igneous rocks, the original voids or body cavities of fossils, and a host of solution cavities in sedimentary rocks have given rise to geodes. The origin of the geodes involves first the origin of the cavity, and second the filling of the cavity.
Significant features of geodes are their sub-spherical shape, their hollow interior, their outer chalcedonic layer and, the inner drusy lining of inward projecting crystals.
Geodes are hollow, globular bodies, varying from couple of centimeters to nearly a meter in diameter (most are 10 to 20 cm).
The mineral matter that fills the cavities comes from ground water passing through the rock. Water contains dissolved matter such as silicon, oxygen, calcium, and carbonate. Under certain conditions, these chemicals precipitate out of the water, forming a solid mineral that is deposited inside the cavities. (This is the same manner in which lime builds up on a stalactite , or on the inside of a tea pot.) One of the most common types of mineral matter filling nodules and geodes is silica, in the form of agate.
Each geode is unique in composition and can only be truly discovered when cracked open or cut with a rock saw. The size and formation of crystals and different shades of color within the crystals make each geode special.
Chotanagpur Plateau of Jharkhand State of India contains numerous varieties of geodes. Villagers sometime confuse it with diamonds filling in rocks.
Most geodes have siliceous outer shells; the physical and chemical durability of silica is responsible for the discreteness and preservation of such geodes. Geodes with outer shells of calcite or iron oxides and hydroxides are less common. Some contain clear, pure quartz crystals, and others have rich purple amethyst crystals. Still others can have agate, chalcedony, or jasper or minerals such as calcite, dolomite, celestite, etc. There is no easy way of telling what the inside of a geode holds until it is cut open or broken apart.
- Pettijohn, E.J., 1984. Sedimentary Rocks. CBS Publishers, India.
- The Encyclopedia of Sedimentology, 1978. ed. Rhodes W. Fairbridge and Joanne Bourgeois. Dowden, Hutchinson & Ross, Inc. Pennsylvania.