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I just watched a video of somebody crushing a Prince Rupert's Drop with a hydraulic metal press.

https://www.youtube.com/watch?v=A6NUNroyUys

The drop dented both the steel bar below the press and the head of his press. It took about 20 tons of pressure to crush the drop.

It must be harder than hardened steel to dent steel like that.

And here is a video of a bullet breaking apart on impact with a Prince Rupert's Drop. The drop not only survived, but was barely dented.

https://www.youtube.com/watch?v=CaYq9sX4zLs

So how hard is a Prince Rupert's Drop?

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    $\begingroup$ There seems to be a distinction between scratch hardness, which the Mohs scale measures, and indentation hardness, which seems more appropriate here. A proper answer might compare the deformation of the droplet before its failure to the plastic (i.e. permanent) deformation of the steel in the press; however most of the information in this particular measurement is about the weaker material, the steel. $\endgroup$ – rob Jan 3 '17 at 16:52
  • $\begingroup$ @rob Thanks for pointing out the difference between scratch hardness and indentation hardness. I will edit the question to refer to indentation hardness. I am still surprised to see that glass is harder than hardened steel. Not just that, but I've seen a video of a bullet breaking apart when it hits a Prince Rupert's Drop, and the glass was barely dented by the bullet. $\endgroup$ – RichS Jan 4 '17 at 5:00

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