Here's a breakdown of their formation:

* Formation: Geodes start as hollow cavities within sedimentary rocks, often formed by gas bubbles trapped in volcanic ash or by the decay of organic matter.

* Mineral Growth: Over time, mineral-rich water seeps into these cavities. As the water evaporates, the minerals crystallize within the cavity, forming the beautiful and often colorful formations we see in geodes.

* Common Minerals: Quartz, amethyst, calcite, agate, and jasper are some of the most common minerals found inside geodes.

Let me know if you'd like to learn more about specific types of geodes or their formation process!