Geodes are spherical rocks which comprise hollow cavities that are lined with crystals. These secondary geological structures occur in particular volcanic and sedimentary rocks. They’re usually found in limestones and basaltic lavas.
They’re of sedimentary origin and form by chemical precipitation. Geodes commonly exist in many formations in the US (mainly in Iowa, Indiana, western Illinois, Missouri, Utah, and Kentucky). Also, they’re common in Mexico, Namibia, Uruguay, and Brazil.
Scientists aren’t entirely sure about the formation of the geodes, but they’ve some theories. So far, the common idea is that they form in the igneous rock gas bubbles. The rock that surrounds the gas bubble hardens, and the minerals (silicates or carbonates or even both) deposit gradually on all existing surfaces.
The dissolved minerals are found in groundwater or hydrothermal. Also, they can form in the spherical hollow regions situated in the sedimentary strata. The common minerals that exist in geodes include celestite, jasper, agate, chalcedony, and amethyst.
Geodes aren’t always dry inside. Water from the period the geodes formed may come out when broken. Geodes differ in content and color as a result of variation of mineral solutions. Quartz crystals which are clear are very common. Purple amethyst crystals decorate the interior of others. Each geode is unique when it comes to the inside orientation and color.
Without further ado, let’s now look at more interesting facts you probably didn’t know about geodes.
1. Crystal Cave is the largest known crystal cave or vug in the world. It is a Celestine Geode 10.7 meters (35 feet) in diameter at its broadest point. It’s situated near the village of Put-in-Bay, Ohio, on the South Bass Island in Lake Erie.
2. The world’s biggest amethyst geode is The ‘Empress of Uruguay’ over 3 meters high (that is 11 feet) and weighing 2.5 tons. It’s situated in Atherton, North Queensland, Australia.
3. Geodes are different from the vugs in that the former were formed as early, rounded, structures within the rocks that surround them and are usually detached intact, while vugs are irregularly-shaped cavities, voids or pockets in a particular formation, usually in breccia or along a vein. Also, geodes are different from the “nodules” in that a nodule refers to a mass of mineral matter which has accreted around the nucleus of the nodule.
4. Geodes can develop in all cavities, but the term is often reserved for less or more rounded formations in sedimentary and igneous rocks. They can form in the gas bubbles in the igneous rocks, like vesicles in the basaltic lavas; or, as in the American Midwest, in the rounded cavities in the sedimentary formations.Once the rock around a cavity hardens, dissolved carbonates and/or silicates are usually deposited on the interior surface. Over time, the slow feed of different mineral constituents from the hydrothermal solutions or groundwater allows the formation of crystals in the hollow chamber.
5. Geodes do have a chalcedony (cryptocrystalline quartz) shell lined internally by different minerals, usually as crystals, mainly pyrite, calcite, kaolinite, millerite, sphalerite, barite, dolomite, celestite, limonite, opal, smithsonite, chalcedony and macrocrystalline quartz that’s the most abundant mineral in the geodes.
6. The coloration and banding of a geode is a result of the variable impurities. Iron oxides impart rust hues to the siliceous solutions. Some geodes contain purple amethyst crystals, but most of them have clear quartz crystals. Still, others can have chalcedony, agate, or Jasper banding or crystals such as celestite, dolomite, calcite, etc.
7. Sometimes geodes and the geode slices are dyed with the artificial colors. Geode samples with strange colors or very unusual formations have often been altered synthetically.
8. Being a state known for geode deposits, Iowa has a geode as its state rock (formally designated in 1967) and has a unique state park referred to as Geode State Park. This celebration of geodes doesn’t stop with Iowa. In Ohio, a huge geode known as Crystal Cave is open for tours.