What Is a Lake Superior Agate?

A Lake Superior agate is a banded variety of chalcedony — very fine-grained quartz — that formed about a billion years ago when this part of the continent was pulling itself apart. Lava poured out across the Superior basin, and as it cooled it trapped gas bubbles. Over an enormous stretch of time, silica-rich water seeped into those empty pockets and laid down layer after layer of quartz on the inside walls. Iron in the surrounding rock stained many of those layers the rusty red and warm orange our agates are known for.

Those agates would still be locked in bedrock if it weren't for the glaciers. Ice sheets ground up the old lava flows and carried the loose agates south and east, tumbling them smooth along the way. When the ice melted, it left them scattered through gravel, riverbeds, road cuts, and — best of all for us — along the beaches of Lake Superior, where waves keep sorting and polishing them for free.

What makes one an agate

• Concentric banding that follows the shape of the old gas pocket, often in reds, oranges, whites, and grays.
• A waxy, greasy sheen rather than a dull, chalky surface.
• Translucency — hold it to the sun and the thin edges glow.
• A hard, glassy feel; agate is harder than a steel knife blade and won't scratch easily.
• A pitted, dimpled skin on unbroken specimens, sometimes called an 'agate rind'.

The common types you'll actually find

Banded agates are the classic: tight parallel or concentric stripes. Eye agates show small rings that stare right back at you and are prized far beyond their size. Husk or paint rock agates have soft, cloudy color rather than crisp bands. Water-level agates formed while sitting still, so their bands lie flat like sediment in a jar. Floaters, which grew suspended in the pocket, band evenly all the way around.

Here's the honest part: most of what you carry home will be beautiful but not agate. Basalt, jasper, quartz, rhyolite, and old chunks of taconite all crowd the same beaches. That's fine. Learning the difference by handling hundreds of wet pebbles is exactly how rockhounds are made, and a good jasper is nothing to apologize for.

One more thing worth knowing: size is rare. A thumbnail agate with clean, bright banding is a genuinely good find on the Eastern UP shoreline, and something fist-sized is the kind of story people retell at meetings for years. Bring it to a club meeting — we'll help you identify it, and we'll be properly jealous.