Geodes are hollow, crystal-lined globular rock cavities found in sedimentary rocks like limestone (calcium carbonate) and dolomite (calcium magnesium carbonate) or in volcanic rocks. In sedimentary rocks, geodes may form by dissolving out of cavities by ground water and redepositing of minerals as crystals which usually point towards the centre of the cavity.
They may also form in pre-existing concretions or in spaces left by expansion of the rock under internal fluid pressure. They often have an outer shell of chalcedony, a crystal lining of quartz, carbonates such as calcite or other minerals, and can be over a metre in diameter.
Each geode is unique in composition and can only be truly discovered when cracked open or cut with a rock saw. The size and formation of crystals and different shades of color within the crystals make each geode special. The rough exterior of the geode gives no indication of the secrets held within its core.
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