🪨 Geodes & Nodules
A geode is a rock with a hollow, crystal-lined cavity. A nodule is the same kind of structure filled solid, often with agate. A thunderegg is a nodule that grew inside volcanic rhyolite.
1 · A cavity forms
Gas bubbles in cooling lava, dissolved pockets in limestone, or hollows left by roots, shells or concretions in sediment make an empty space.
2 · A rind hardens
Silica-rich fluids coat the cavity wall with a tough skin of chalcedony. This is why geodes weather out of softer rock as round "rocks".
3 · Crystals grow inward
Over thousands to millions of years, mineral-rich groundwater seeps in. Quartz, amethyst, calcite, celestite or other minerals crystallize from the wall toward the centre.
Geode, nodule or thunderegg?
If the space stays partly open, it is a geode. If it fills completely, often with banded agate, it is a nodule. A thunderegg forms in rhyolite and usually has a star-shaped agate core.
See also Stalactites: drip-grown columns from the same cavities, including amethyst 'flower' slices.
%202%20(32754410355).jpg?width=240)
%202%20(32612594515).jpg?width=240)
