A geode is a round or oval-shaped rock that looks ordinary and dull on the outside but contains a hollow cavity lined with beautiful crystals on the inside. When you crack open a geode, you might find sparkling quartz crystals, purple amethyst, or other colorful minerals that formed over thousands of years.

Geodes form when gas bubbles get trapped in cooling lava or when water seeps through hollow spaces in rock, depositing minerals drop by drop. The crystals grow inward from the walls, creating stunning patterns that no one can see until the rock is broken open. Some geodes are small enough to hold in your hand, while others are huge, with cavities large enough to walk into.

Finding and breaking open geodes is popular with rock collectors and kids who love geology. You can buy uncracked geodes at science museums or rock shops. The excitement comes from not knowing what's inside: each geode is unique, and you won't discover its hidden treasure until you carefully split it open with a hammer or rock saw.