Obsidian: The Sharpest Rock on Earth (and in Surgery)

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In this third Volcano Minisode, Katy digs into one of the coolest things a volcano has ever made: obsidian—a rock so sharp it's been shaping human history for 30,000 years and is still used in modern surgery. 🔪🖤

🌋 What exactly is obsidian and how is it formed?
⚡ How can lava turn into volcanic glass in a flash?
🩺 Why are obsidian scalpels sharper than steel—and still used today?
🛡 How did ancient people turn this into tools, weapons, and even mirrors?

From Stone Age scrapers to eye surgery scalpels, obsidian proves that volcanoes don’t just destroy—they create tools that changed the course of human evolution.

👉 This is episode 3 of 6 in our Volcano Minisodes series—bite-sized, bizarre, and bursting with molten-hot science facts.

🎧 Listen now to discover why this rock deserves way more credit.

Transcript

Laura: [00:00:00] Hello and welcome to Wildly Curious, a podcast that tells you everything you need to know about nature, and probably more than you wanted to know. I'm Laura.

Katy: I'm Katy, and today is the third episode where we're talking about volcanoes and the surprising things they've made, buried and accidentally made cooler. So my first week I talked about things that were buried.

Laura: Then I

Katy: Laura, you were? Yeah. Yeah. And things that were, yeah, I guess things were made too, but mine is more of a made buried, I guess, and a little cooler.

So in this one, I, I, this one I really thought that you were gonna pick, I will have to give a shout out to my son, Luke, because the only reason why I was like, oh, this one, because he talks about it in Minecraft so much. So this one I'm talking about obsidian.

Laura: Ooh. Hey, that's cool. I would not have thought of this angle.

Katy: Really because I was like, man, Laura's gonna, clearly, Laura's

Laura: I mean, I love a good a rock and obsidian is so beautiful.

Katy: If you don't know Laura and I, we blindly go into these, we pick a [00:01:00] theme and then each of us, we pick what we wanna talk about.

We send it to a mutual third party, thank you, Taylor. And then she makes sure that we don't repeat anything. And so Laura and I don't know what the other one's talking about. So when I sent mine to Taylor, I was like, all right, if Laura talks about this one, here's, yeah, here's my backup. And he didn't. So, all

Laura: I'm glad we didn't have any overlap. I was also like, , I don't know, maybe Katy, I'll about this one.

Katy: Nope. Obsidian. So what is obsidian? It's volcanic glass, essentially. And it's definitely nature's, I would say sharpest material by far.

So this isn't just any rock , this is made when lava cools so fast, 

it doesn't have time to even crystallize. It skips the whole in-between, solidifying, getting all of its mo molecules all lined up. It completely skips that and becomes glass. So here's how it works. Lava erupts, usually the more viscous, the sticky, slow moving silica rich kind, and instead of cooling [00:02:00] slowly and forming, chunkier crystals, it slams into cold air, or better yet, water.

When that happens, it quote unquote freezes instantly, but it doesn't freeze.

Laura: of like ice looking.

Katy: Yes, yes. 

So because it cools so fast, obsidian, like I said, it's a non crystalline. That means that there was no time for the, or the orderly mineral growth.

No flashy crystal structures, just smooth, hard, shiny black glass.

Laura: Right, which is interesting. There's so many different types of igneous rocks made from heat, but this is the kind that's like, boom.

Katy: But yeah,

Laura: get with no growth.

Katy: mm-hmm. Like a flash freeze almost. Sometimes it's dark, green, brown, or even like the edges of it can be rainbowy sort of color. , if there were bubbles or impurities, those can still be caught too quick enough. So you can find this. Normally at the edges of the lava flows at the top of the lava domes or anywhere molten [00:03:00] rock suddenly got shut down, especially if it meant water, like I said.

And because it's so high in silica, like more than 70%, which is why it goes so slow. It's glossy, brittle, and ridiculously sharp when it breaks. And when I say sharp, like absurdly, absurdly, like sharper than your kitchen knife. Sharp. So when obsidian breaks, it doesn't crumble like a normal rock, because again, it's all like how it has to flash.

Yeah. It shatters in a pattern called a. Can Troy can choroidal fracture.

Laura: Hmm.

Katy: Anyway, that's just a fancy way of saying it breaks into perfectly curved shards with edges so thin Sometimes they can just be a few molecules thick, which is mind blowing

Laura: I actually, ironically, for any of you who are watching, I have obsidian, arrowheads literally in front of me on the desk.

Katy: what the heck.

Laura: Why you might ask, I don't know. Mom gave them to me, [00:04:00] but like I,

Katy: all, I love your mom,

Laura: yeah, sorry. What do you

Katy: what are the

Laura: Arrowheads and what are the chances? But so cool. And there's a guy on TikTok who makes obsidian arrowheads that I like to follow, but that guy.

First of all, there's usually blood involved because of how

Katy: imagine there not

Laura: But he has to have leather guards on his legs and leather gloves so that when he makes these spearheads and arrowheads, he isn't slicing himself to ribbons.

Katy: Yes. , like you said, Arrowhead, but they've made, they make surgical tools from these, this is like the peak of surgical tools. So they make obsidian scalpels,

Laura: That's what I thought. I thought they still did that. A modern tool

Katy: Yes. A modern tool

Laura: of stone, like

Katy: Yes, for eye, for procedures that are like eye surgery and tissue transplants, because obsidian blades are up to five times sharper than what we can ever make steel, which means there's less damage to the tissue, less scarring, and much, much cleaner healing because [00:05:00] it's just like such a

Laura: That is so amazing. It's so amazing that we have probably been using this for very similar things since

Katy: So, they've tracked it back more than 30,000 years that we've been using obsidian.

Laura: this is sharp.

Katy: Yes. Let's just keep going with it

Laura: and then we're still like we, this is 2025. We use computers and lasers to do surgery, but we also still use rocks. I'm like, it's so cool.

Katy: Yeah. And it's just crazy. So the Stone Age folks used it for knives, like Laura said, arrowheads, like little scrapers, for, making the clothing, how you have to scrape, like all this, uh, hide everyth. Yeah. Aztecs used it for weapons and mirrors because it does get so shiny.

 The Mayans used it in rituals, which I'm like, that took a violent turn. But the obsidian wasn't just a local thing. It became one of the earliest forms of long distance trade too, which would make sense because you figure you had to be, [00:06:00] yeah, you gotta be near a volcano. But it was so valuable because it's so sharp.

Laura: we weren't making anything like that back in the day.

Katy: No. Heck no. And so people would travel long distances with the obsidian because that's the good stuff. You know what I mean? Like it could do anything and everything.

Laura: I bet, I bet 

people made razors out of it.

Katy: oh yeah, ev like anything that cut, shave, anything like that. Obsidian. So again, the whole, this whole mini Sears are talking about volcanoes right?

And so part of it, I was talking about, they make cool things. And this is definitely one of those really cool things that it makes. So again, it's just this is shows, the obsidian is like what happens when the destruction. Of a volcano freezes like mid motion, no crystals, no minerals, just like boom, instant transformation into something insanely sharp.

So it might be a rock, but it's truly one of the, there's very few, I shouldn't say very few, like diamonds, of course, that shaped economy and everything for [00:07:00] very negative reasons. But this is one that has shaped culturally for like 30,000 plus years. But in a positive life that I don't think enough people really think about.

Yeah, know

Laura: think about obsidian, but you're right. It's totally shaped human history

Katy: yes.

Laura: could do with those tools.

Katy: That we never could do before. So yeah. So that's obsidian guys,

Laura: rock, as you can

Katy: right? That Lord just happens to

Laura: It's right in front. I don't know if it's real obsidian, but it sure feels and looks like it.

Katy: Yeah. Crazy. All right guys, well make sure you go check us out on YouTube and Facebook and, we will talk to you then next

Laura: See you guys.