Geodes are rocks that are plain on the outside but can have beautiful crystals on the inside. In the Greek language, geode means “shape of the earth”, and geodes are round like earth or oblong like an egg. They can be a couple inches or several feet in size.Тhe largest crystals can take a million years to grow.Which minerals end up as crystals in a geode varies by location and conditions such as temperature, acidity, and the type of rock the geode forms from. For instance, quartz crystals are most common in igneous geodes. Many other minerals can be found inside geodes, too.
They are themselves of sedimentary origin formed by chemical precipitation. Geodes are essentially hollow, vaguely spheroid to oblate masses of mineral matter that form via either of two processes:
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by the filling of vesicles (gas bubbles) in volcanic to sub-volcanic rocks by minerals deposited from hydrothermal fluids or
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by the dissolution of sedimentary nodules or concretions (that were deposited syngenetically within the rock formations in which they are found) and partial filling by the same or other minerals precipitated from diagenetic water, groundwater or hydrothermal fluids.
What causes all the colors?
Geode banding and coloration is the result of variable impurities. Iron oxides will impart rust hues to siliceous solutions. Most geodes contain clear quartz crystals, while others have purple amethyst crystals. Still others can have agate, chalcedony, orjasper banding or crystals 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.
Geodes and geode slices are sometimes dyed with artificial colors. Samples of geodes with unusual colors or highly unlikely formations have usually been synthetically altered.
wikipedia
Geodes are found throughout the world, and many are concentrated in deserts.
Volcanic areas are a common place to find geodes.
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