From the outside, a geode often looks like an ordinary rock. Crack it open, however, and you may discover a glittering crystal-lined chamber that has been forming for millions of years. But why are geodes hollow in the first place? Learn how these fascinating formations develop, why crystals grow inside them, and what determines whether they remain hollow or become completely mineral-filled.

What do Herkimer Diamonds, Lake Superior Agates, Oregon Sunstone, and Bisbee Turquoise have in common? They're among the most iconic rocks, minerals, gemstones, and fossils found in the United States. In celebration of America's 250th birthday, we're taking a coast-to-coast look at ten legendary specimens that have inspired collectors, rockhounds, and gemstone enthusiasts for generations.