Geode
Definition
A hollow, rounded rock containing crystals that grew inward from the walls. Often lined with quartz or amethyst crystals.
Example
Amethyst geodes from Brazil can contain spectacular purple crystal formations.
Hidden Treasure Chests
A geode is a hollow, subspherical rock with crystals lining the inside wall. From the outside, they look like ordinary, ugly lumps of rock (nodules), but cracking them open reveals a sparkling cave of crystals. They are one of nature's greatest surprises.
How They Form
- Volcanic Geodes: Gas bubbles form in cooling lava. Once the lava hardens, groundwater seeps into the empty bubble cavity, depositing silica which crystallizes into Amethyst, Citrine, or Agate.
- Sedimentary Geodes: Form in cavities in limestone or shale, often where an organic object (like a shell or tree branch) has decayed, leaving a void that is later filled with Calcite, Celestine, or Quartz.