Why Diamonds Can Shatter

The story

Here is a riddle. Diamond is the hardest natural material on Earth. No other natural material can scratch it. Only another diamond can. And yet, if a diamond falls onto a hard floor at just the wrong angle, or is struck with an ordinary steel hammer, it can crack, and even shatter into pieces. How can the hardest thing in the world break?

The riddle unravels when you learn a secret about materials. Strong is not one thing. Strong is several different talents, and a material can have one without the others.

The first talent is hardness. Hardness is about scratching. Press two materials together and rub, and the softer one gets the scratch. Diamond out-scratches every other natural material on Earth. It scratches steel, it scratches glass, it scratches rock, and none of them can scratch it back. That is what hardest truly means.

The second talent is toughness. Toughness is about surviving a blow, soaking up a sudden knock without cracking. And here the ranking turns upside down. A rubber ball is soft, yet remarkably hard to break. It can squash, stretch and spring back, over and over, without tearing or cracking. Glass is much harder than rubber, but one unlucky fall can leave it in pieces. Hard and tough are different talents, and they often pull against each other.

Now look inside the diamond. Its atoms are carbon, holding hands in a perfect, rigid pattern, every atom gripped firmly from every side. That firm grip in every direction is why nothing else can scratch it. But perfection carries a price. Running through that neat pattern are flat layers where the atoms' grip is a little weaker, like the grain in a piece of wood. A sharp knock at the wrong angle finds one of those layers, and the crack runs along it, fast and clean. Diamond cutters have long known those hidden layers well. In the old way of working, a cutter would prepare a tiny groove, rest a blade in it, and give one careful, well-aimed tap, and the diamond would split smoothly along its layer, ready for shaping and polishing.

Steel plays the opposite game. Struck hard, it bends a little and absorbs the blow. It is far softer than diamond, yet far tougher. That is the answer to our riddle. The hammer does not out-scratch the diamond. It out-toughs it.

So when someone wants to know the strongest material, you can ask the cleverer question back. Strong against what? A drill that must chew through rock wants hardness, which is why some rock-cutting tools carry tiny diamonds in their edges. A hammer that must take blow after blow wants toughness, which is why it is made of steel and not of gems.

The next time you hear that diamonds are unbreakable, you can share the secret. The hardest natural material on Earth out-scratches everything around it, and still needs to be set down gently.