Amazing Lake Superior agates—semi-precious stones made up of concentric circles of microcrystalline quartz reddened by iron—are the oldest on earth.
Size: Pea-sized to up to more than 20 pounds.
Formation: Formed during lava eruptions in the area of what is now Lake Superior. Bubbles of gas were trapped in the molten lava; after the lava cooled, water seeped into the holes, depositing iron, quartz and other minerals in beautiful layers. During the Ice Age, glaciers swept across the Lake Superior region, releasing agates.
Claim to fame: Lake Superior agates are the oldest agates on earth, formed 1.2 billion years ago. The next oldest are the Montana moss agates, a mere 370 million years old.
The most prized: Highly contrasting red and white bands (called candy stripers), eye-like patterns (eye agates) and hologramlike appearance (shadow agates).
Hunting grounds: Comb any pebbly beach on the south and west shores of Lake Superior, but the beaches near Grand Marais are famed for their agates.
What to look for: A pitted or peeled texture. A glossy, waxy appearance on chipped surfaces. Rust- and yellow-hued iron oxide staining. Translucence produced by their quartz content, causing a glow easily observed on sunny days, or when the rock is backlit with a flashlight.
When to look: Best hunting is right after a storm with churning waves.