Rock with a crystal-filled cavity

GEODE

Think of a hollow, rounded stone that looks ordinary outside but sparkles inside—often split open to reveal stunning mineral formations like amethyst or quartz.

A geode is a round rock that looks ordinary on the outside but hides a hollow center lined with sparkling crystals like quartz or amethyst. The clue describes it directly—"rock with a crystal-filled cavity"—since that’s exactly what makes geodes special. No wordplay here, just a straightforward definition for a natural wonder often found in volcanic or sedimentary rock.