A geode is a geological formation consisting of a hollow rock, often a nodule or an irregular cavity in a sedimentary or volcanic rock, lined with crystals.

Here's a breakdown:

* Formation: Geodes form when gas bubbles or cavities are trapped in volcanic rock or sedimentary rock.

* Crystal Growth: Over time, mineral-rich water seeps into these cavities. As the water evaporates, minerals crystallize on the inside walls of the geode, forming beautiful and intricate patterns.

* Appearance: Geodes can vary greatly in size, from small pebbles to large boulders. The most well-known geodes are those filled with amethyst crystals, but other common minerals include quartz, calcite, agate, and opal.

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