Like many children, I was captivated by museum gift shops, especially the shelves of glittering geodes. Crack one open and you’re rewarded with a surprise display of crystals hidden inside. Those pocket-sized treasures, though, are nothing compared to the largest geode in the world – one so vast it could swallow the entire gift shop whole.
The story begins in 1897 on South Bass Island, Ohio, where German-American winemaker Gustav Heineman had set up a vineyard. When he ordered a well to be dug to supply water for his vines, workers broke into a cavern 12 metres down. Instead of solid rock, they found a cave lined with enormous crystals.
The cave’s origins trace back hundreds of millions of years. During the Silurian period, 430 million years ago, this part of North America was covered by shallow seas. Layers of sedimentary rock formed, including lenses of the evaporite mineral anhydrite (calcium sulfate). Fast forward to the end of the last Ice Age: meltwater from retreating glaciers and nearby Lake Erie seeped through the rocks, dissolving the anhydrite and leaving behind empty cavities.
Normally, these spaces might become crystal-lined geodes filled with quartz or amethyst. But here, something unusual happened. The groundwater was rich in strontium. As it interacted with the dissolving anhydrite, calcium ions were replaced by strontium, forming celestine – pale blue, glassy crystals of strontium sulfate. Over thousands of years, they grew to extraordinary sizes, some more than a metre across, filling the cavern with their sky-coloured sparkle.
The cave, however, didn’t remain untouched. In the early 20th century, miners extracted around 150 metric tons of celestine crystals, not as souvenirs but as a source of strontium for the fireworks industry, where it produced a brilliant crimson flame. The removal enlarged the cavern to its current size – 11 metres across and tall enough to stand in.
Recognising an opportunity, Heineman’s son Norman opened the cave to visitors in 1919. The timing was fortuitous: during Prohibition, when most Ohio wineries were forced to shut, ticket sales to the “Crystal Cave” (along with grape juice) kept the business alive.
Today, more than a century later, the Heineman Winery and its glittering celestine cavern still welcome tourists, making South Bass Island home to both award-winning wines and the largest geode on Earth.
This year I’ve had the pleasure and privilege of writing a series of mineral-focused scripts for SciShow’s limited-run Rocks Box subscription. It’s been such a joy being able to nerd out about rocks and minerals. I’ll write about anything, but geology will always be my first love. Watch this space for many more rocks-related updates!
Watch the full video from SciShow here:
Leila Battison 
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