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An agate nodule is a type of geological formation where agate, a variety of chalcedony (which is a microcrystalline form of quartz), forms within a nodule. Here's a more detailed explanation:

Formation: Agate nodules typically form in volcanic rocks, especially in cavities or vesicles left by gas bubbles in lava flows, or in sedimentary rock layers where silica-rich solutions seep into voids or around organic matter. Over time, these solutions deposit layers of silica, which eventually harden into agate.
Structure:
Nodule: The term "nodule" refers to a rounded mass embedded in rock. In the case of agate, the nodule often has a concentric or banded appearance due to the way silica layers are deposited.
Bands: Agate nodules are famous for their colorful, concentric bands, which can vary widely in color due to different mineral impurities present during formation.
Appearance: Agate nodules can be quite varied:
External: Often, the outside of the nodule is less colorful, sometimes covered by a layer of rough, less translucent rock material.
Internal: Once cut or polished, the inside reveals the banded, translucent to semi-transparent agate with patterns that can be quite intricate, including eye-like formations, landscapes, or even dendritic patterns if manganese or iron oxides are present.
Uses:
Jewelry and Ornamentation: Due to their beauty and durability, agate nodules are cut into cabochons for jewelry, sliced into slabs for decorative items, or simply polished as display pieces.
Metaphysical: Some cultures attribute healing or spiritual properties to agate, using them for various forms of alternative therapy or as protective amulets.
Locations: Agate nodules can be found in numerous places around the world where conditions for their formation exist, including Brazil, Uruguay, Mexico, the United States (notably in Oregon for its famous thundereggs, which are a type of agate nodule), and parts of Europe.

Remember, while collecting agate nodules can be a hobby, it's important to do so responsibly, respecting private property and environmental regulations where applicable.