![]() These layers often occur within cavities that have already been partially lined with agate. Horizontal Banded Agate - A less common form of agate banding where the agate deposits in horizontal layers. This type of formation can occur in both spherical cavities or when cracks in the host rock are filled with a silica-rich fluid (seam agate). As the space fills, the layers of agate follow the shape of the surrounding rock, in some cases giving the specimen unique shapes as the layers shrink. The microscopic crystals deposit in layers that can vary in color dependent on the impurities present during formation. This is the name for agate that deposits along the walls of a cavity. Wall-Lining Banded Agate - A very common form of agate. If the water is mineral rich and/or has picked up minerals from the surrounding host rock, crystal formations can occur, in this case agate. Water can then permeate through these cracks through both ascending (hydrothermal) and descending fluids. Agate within Seams or Vugs - Cracks occur within rock formation for a variety of reasons (tectonic plate movement, landslides, earthquakes, etc.). These agate formations occur throughout the world in locations where volcanic activity has created these environments, explaining why agate nodules and geodes can be found on every continent. Mineral-rich water then made its way into the hollow cavity and provided the necessary minerals were present, agate could then form within the cavity. Eventually the gas escaped through the rock following the rock solidifying and cracking. They moved their way up through the rock as it was still in a viscous state. These bubbles within the rock formed as a result of rapid gas formation from expansion of volatile components. Agate within Nodules or Geodes - Nodular or geodic agate formations occur in bubbles within the host rock that were created during volcanic activity. In some cases this can result in concentric banding within a nodule, or layered formations within cracks (seams, vugs, veins, etc.) between rocks. Depending on impurities present during formation, the agate will exhibit varied coloration as the impurities work their way into the crystal lattice. This deposition occurs molecule by molecule, forming tightly packed fibrous microcrystalline quartz crystals that line the contact walls of the cavity. Agate is a type of chalcedony (variety of quartz) that forms when a silica-rich solution fills an open cavity within rock and begins to deposit crystals evenly along the walls.
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