What is the most durable rock on earth?

Craig McClarren
4 min readMar 29, 2021

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Without a doubt, the title holder for “World’s Toughest Rock” (not a mineral, but a rock) has to go to Banded Iron Formation, or at least to a related variety of ironstone (there are several).

Anyone who’s ever dealt with Banded Iron Formation, which we usually refer to as BIF, knows that it’s a beautiful rock that is also an absolutely gnarly bitch to break or cut. The reason is pretty simple: it is composed of alternating layers (bands) of quartz, which is already a pretty hard material, and hematite. Hematite is Fe2O3. It is hard. It is iron. It is not your friend if you’re trying to do anything with it.

I took a few photographs at Jasper Knob in Ishpeming, Michigan, billed as the world’s largest gemstone. It is composed of BIF. That BIF is composed of layers of red hematite, silver specular hematite and quartz minerals. It is gorgeous stuff and since that’s a world class site, I didn’t even bother trying to whack any samples off of it. There are other places to collect. Here’s what it looks like:

There’s not much quartz visible here… it’s really just the alternating layers of red and specular hematite you can see.

It can be a little brittle, so if you’re lucky enough to just find a thinnish sheet of it lying around with just a few layers, it can be broken. If you’ve got a big block of the stuff, though, you’ll never break it. You just won’t. It is harder than pure iron. You will do more damage to the rock hammer that you’re beating it with than you’ll actually do to the rock itself. Trust me, I know this. I was president of the Michigan Tech Geology Club half a lifetime ago and we visited the iron country all the time. You can’t pound your way through it.

As a fundraising exercise, back when I was an undergraduate and president of the Geology Club, we made and sold rock clocks. Using a rock saw, we’d cut slabs of rock, coat them with resin or lacquer (I can’t really remember what it was now), drill a few holes through, put some numbers, clock hands and a little battery motor on the back and bam: you’ve got a gorgeous clock that we’d sell to fund our club. Well, someone collected a couple of enormous blocks of BIF and suggested we try it out. I was skeptical, but I told him that if he wanted to run it through the rock saw, himself, he was welcome to. It took him more than two hours to cut a single slab. The rock saw blade was absolutely useless after that- that BIF ate through the blade like it was made of tin. Sheer determination led to him getting two BIF slabs cut before the department put a stop to it, with very justifiable worries about its equipment. They made gorgeous clocks, but it was insane to try in the first place.

Also, the banded iron formation you see in these photos is 1.8 billion… that’s billion with a “b”… years old and is virtually unchanged over that time. Now, that’s durable!

Edit

I was asked, after writing this, to post a photo of the banded iron formation rock clock. Sadly, those were made in 2002, well before digital photography was common and I haven’t any photos of those. I do happen to be visiting my parents’ house just now, to whom I gifted an epidote clock at that time. I took a quick photo just now and this is what it (epidote, not BIF) looks like:

Amazingly, that little battery motor on it still works after all these years!

Here are some pictures of Jasper Knob, better displaying its amazing soft sedimentary deformation. I’ve adjusted the contrast on the photos to better show it, but the colors are still very much what you see when you’re there in person.

Those are my wife’s feet for scale :)

Pretty geologically awesome stuff!

Image credit: Craig McClarren

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Craig McClarren

Geologist, a lover of all science, father of a young child, published writer on Forbes and Mental Floss