I used to hang out a lot at the Rock Shop. You know the store by the freeway where you can crack rocks that look like regular dull rocks but actually have this crazy dyed crystal stuff on the inside?
Amy is talking about geodes, which are spherical or oblong rocks filled with crystals. The crystals form when mineral-rich groundwater or rainwater seeps through porous rock, leaving behind mineral deposits. The minerals layer over time and eventually fill the rock cavity with crystals, though the type varies by location, temperature, acidity, and type of rock. Quartz crystals are some of the most common, but silica, hematite, dolomite, calcite, and “fool’s gold” are some other common types that might appear. Geodes are frequently cut in half, polished, and sold as souvenirs.
Here are a few examples: