FORMULA: CaCO₃

LOCATION: Ogdesnberg, New Jersey, USA

This rock you’re holding is Franklin ore, and it’s old. Like, really old. So old that studying it has allowed researchers to learn about some of the very first organisms to live on our planet. Part of this rock contains the fossils of stromatolites, mats of single-celled organisms that built up in shapes almost like stalagmites over hundreds of years. Then, these layers turned to stone, and were preserved for more than a billion years.

And studying these fossils also gives researchers clues about how the Appalachian Mountains, some of the oldest in the world, formed. That story is way more complex than it might seem. See, the Appalachians didn’t just form in one go. There were five different mountain-building events that occurred over hundreds of millions of years. All of which we can understand thanks to this unassuming little rock!