I just watched a video of somebody crushing a Prince Rupert's Drop with a hydraulic metal press.
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.
So how hard is a Prince Rupert's Drop?