How Hard Are Diamonds? | Are They Really That Hard?

(Last Updated On: June 7, 2022)

You’ve probably grown up being told that diamonds are forever, or that the diamond industry is built on human rights abuses and child labor. You know. One of the two. What you’ve probably also heard is that diamonds are some of the hardest things out there. But you’re probably aware that everything has its limits, so while we contemplate the limits of diamonds–how hard are they really? 

What Is Hardness?

“Hardness” is something that we all intuitively understand, you know what it means when someone says a particular item is “hard.” But there is a technical definition of the term, which we’re going to have to apply when asking “how hard are diamonds, really?”

You’ve probably heard of the Mohs scale of mineral hardness–which was introduced in 1822 by a guy named Friedrich Mohs. It’s a qualitative scale, and only one of the multiple ways in which hardness is measured. While the Mohs scale isn’t known for being precise, it is a nice starting point. The idea behind the Mohs scale essentially asks the following question: “can this thing that I just found scratch this other thing I have?” This scale compares the hardness of two objects, if object A can scratch object B, then object A is harder. On this scale diamonds are considered the hardest object, with a score of ten, and the softest mineral is talc–with a hardness of one. When you’re doing a scratch test (like the Mohs scale measures), the only thing that can scratch a diamond is another diamond. 

Other Kinds of Hard

Beyond scratch hardness there’s indentation and rebound hardness. Indentation hardness measures how likely something is to deform when compressed. It asks the question “how hard do I have to press down on this thing before it begins to deform.” Hardness here is quantified by pushing a sharp object into something and measuring the indentation left behind. There are multiple scales used to measure hardness, like the Rockwell or Vickers test. Many indentation tests use a diamond-tipped point to make an indentation, and beyond that diamond hardness here is exceedingly difficult to measure. Values of diamond indentation hardness vary wildly, because indenters can break when pressing into diamonds or the diamonds will simply fracture before an indentation is made.

There’s also rebound hardness, which measures how much sometimes bounces back when smashed. It’s similar to asking how elastic something is–if something cracks or shatters that’s not deforming. You’ve just destroyed the object. This smashing is done by a diamond-tipped hammer, and like indentation hardness diamonds don’t really want to deform. At all. For a long time it may have seemed impossible, but research done in 2020 suggests diamonds do deform at the nanoscale

What Makes Diamonds So Hard?

The thing about diamonds is that they’re just carbon. If you’ve taken high school chemistry, you know carbon has some pretty unique properties–like having four electrons and basically always making four covalent bonds. When carbon makes a crystal structure, they form tetrahedrons, which is like if you made a pyramid but the base was a triangle. They look like this:

This tetrahedron is repeated over and over again in a lattice within a diamond. This basically means all bonds are equally strong, and because these bonds are covalent bonds they’re all really strong. It’s also why it’s super hard to melt diamonds. 

You might hold up graphite and question this claim. Graphene, like diamond, is made up of just carbon. But it’s inside all your pencils and you can break that stuff easily. The reason why graphene is weaker than diamond is, again, its structure. Instead of a bunch of interlocking tetrahedra, graphene makes a bunch of flat, hexagonal layers. So these layers can come off quite easily.

How Hard Is It to Smash a Diamond?

Not too hard, actually. It comes down to cleavage. Go back to the example of graphite. The bonds within one layer of graphite are super strong–but ones the ones between layers are far weaker. This makes it easier to “cleave” graphite between layers. Diamonds don’t have a single point to cleave from, the tetrahedral pattern makes for four planes of weakness–instead of the single point for graphite. So if you hit a diamond along one of the four planes, you can cleave it–this is a weakness that’s often exploited when diamonds are cut for consumers

Speaking of diamonds, see if you know where they come from here.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.