I clearly remember learning in my sixth grade science class that diamond is the hardest substance on earth. As a kid, I used to fantasize about diamond armor and diamond weapons that would be incredibly powerful as they could never break, and when I grew older I heard about the power of diamond drills and saws that could cut through concrete. With this is mind, I was extremely surprised to see what happened in this recent video made by the Hydraulic Press Channel.

They appear to be shattering a diamond into dust by applying pressure and the metal plate underneath hardly seems affected at all. Could it be a hoax or a trick, or are diamonds not quite as tough as I thought?

Ultimately my science teacher did not tell me everything I needed to know. It turns out diamonds actually are incredibly hard, they are just not all that tough. Hardness is a measure of how resistant a material is to scratching or warping due to pressure. Materials like diamond, ceramic and glass are very resistant to being bent or chipped away, but they will break more easily than steel when pressed upon with enough force because of their resistance to changing form. It explains why you see very impressive sculptures made from metal and stone but rarely see intricately carved diamonds or other gemstones, the material is too hard and will shatter more easily than it will be cut apart. This explains how the diamond was shattered in the video, because it was so hard it was unable to bend when compressed and fractured when the pressure became too great.

The tendency to shatter shows a lack of toughness, a measure of how easily a material will break when force is applied. Very tough materials might be more easily scratched or bent than diamond but they will be more resistant to breaking apart when under a lot of pressure. Some of the toughest substances are also incredibly easy to bend and warp, such as spider silk and threads like Kevlar. There are also materials that are both incredibly hard and tough, in this video (at around 8:03) you can see a piece of tungsten carbide that is able to warp the metal underneath it and suffers no damage whatsoever when put under similar pressure to the diamond. Diamond is still the hardest substance on earth, but it still can be shattered with enough force.