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If you were really into gift shops as a kid, you probably loved to look at all the shiny geodes. But those little geodes are nothing compared to the mother of all geodes, found is in Put In Bay, Ohio. So let's talk about how this geode formed and the gorgeous sky-blue celestine crystals that we find inside it.

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As a kid, there was basically nothing better than going to a gift shop.

The toys and games and stuff were all great, but the best part for me  was always the weird rocks. Especially the geodes.

But what may be the world’s biggest geode is big enough to fit its own gift shop inside! It’s more than 10 meters across, and filled with gigantic crystals  of a mineral called celestine. And the story of how those crystals got there is many millions of years in the making.

So let’s crack open the mother of all geodes, and see what we find! [Intro music] Our rockstar geode is in Ohio, right off the shore of Lake Erie. It was discovered back in 1897 by, o f all people, a winemaker. This German-American entrepreneur Gustav Heineman had set up a winery on the fertile soils of South Bass Island in Put In Bay.

When he commissioned a well to be dug on his land to supply his grape vines, the workers stumbled across something unexpected about 12 meters deep. They’d broken through into a small cave, filled with huge crystals. The fact that there was a cave in this region isn’t all that surprising, but to understand why, we need to do a little time traveling.

At the end of the last ice  age, about 14,000 years ago, glaciers gradually retreated from North America, exposing the ice-scoured rocks underneath and creating the Great Lakes. In northern Ohio, this revealed layers of sedimentary rocks that were formed at the bottom of the ocean, 430 million years ago during the Silurian period. There were shales and carbonate rocks, but between some of the layers there were also sometimes pockets  of a mineral called anhydrite.

Anhydrite is made up of calcium sulfate, and it’s what’s known as an evaporite mineral, because it forms when sea water… you guessed it, evaporates. It’s kinda like the salty crust that’s left on your clothes  after a day at the beach, except on a much, much larger scale. So when the melting ice exposed  those ancient sediment layers, cracks in the surrounding rocks allowed water from Lake Erie to trickle all the way down into those anhydrite pockets.

And just like a shower dissolves  the salt off your swimsuit, when the lake water reached the anhydrite, that anhydrite dissolved, leaving gaps in the rock that became caves. * There are at least 30 of these caves on South Bass Island alone. But while these caves might be common, caves full of crystals are not! So here’s how a run-of-the-mill cave gets bedazzled by nature.

You start with a hole or an open space in a rock. These are most commonly found in volcanic rocks, because the lava sometimes solidifies with air bubbles still inside it, but they can be just about anywhere. If mineral-rich water finds its way into that hole and can’t drain back out, it can sit around for a long  time and slowly evaporate, which allows the minerals to form into crystals.

The basic idea is pretty  similar to making rock candy, except that it’s inside a rock  bubble and not on a stick. And also probably not tasty. The crystals grow over thousands  or even millions of years, starting on the cavern walls and growing inwards, pointing towards the center.

So the bigger the crystals, the older your geode is. And the kind of crystals you find in there depends on the specific minerals that were in the water in the first place. The most common ones are quartz-based minerals like clear quartz, purple amethyst, or bubbly-looking chalcedony, all of which you’ve probably seen in gift shops.

But occasionally rarer mineral  ions can make it in there, and contribute to something special, like the delightfully named celestine. Celestine, also sometimes called celestite, grows as chunky crystals  with glassy or pearly luster. Composed of strontium sulfate, they’re usually pale blue, like the sky, hence the name.

They’re typically found in  sedimentary environments, so finding them inside a geode is already rare in the first place. It took a series of unique  circumstances at Put In Bay to produce this dazzling celestine geode. First, it needed a sulfate-bearing mineral already in the rock, which is that anhydrite we talked about.

Then, strontium-rich groundwater  had to permeate into those rocks. So a pocket full of calcium sulfate, AKA anhydrite, got saturated with strontium-rich water. And over time, the calcium in the anhydrite got replaced by strontium from the groundwater, creating strontium sulfate crystals, AKA celestine.

Now, while that calcium sulfate  dissolves relatively easily, strontium sulfate is far less soluble, meaning that once it started to grow, the water that was left in there wouldn’t dissolve it away again. So that’s how we ended up with a cave full of those pretty pale-blue celestine crystals. Testing has shown the  celestine in the Crystal Cave grew over the course of several thousand years, between the end of the ice  age and about 6,000 years ago.

That gave the crystals plenty of time to grow, and boy did they! Some of the crystals in this  cave are up to a meter wide! And more than 100 years after it was discovered, both the winery and the world’s biggest geode are still thriving as a tourist  destination in northern Ohio.

I myself would love to go So the world’s weirdest geode  and all its pretty blue crystals remain available for anyone to go see. But our Rocks Box subscribers won’t need to go anywhere  to get their celestine fix, because it’s coming to you! Every month, Rocks Box subscribers get an ethically sourced, high-quality mineral specimen  sent right to their doors.

Plus, we make a cool video like this one to tell you all about what you’re getting! If all that sounds like something you’d be into, head on over to SciShow. Rocks to sign up.

And hurry, we do not have unlimited numbers of these specimens and it fills up fast! Thank you for watching. [Outro]