Diamond is the hardest known, naturally occurring material, but this does not make it supernatural. if you had that sheet of diamond, somehow, you'd be able to pick it up and snap it in your hands with no trouble.
If you had the option of making an I-beam for your building out of pure diamond or steel, you would pick steel, because the diamond would quickly shatter.
In mineralogy, hardness is used to reference a mineral's ability to scratch another mineral, and also indentation, its resistance to compression.
The diamond lattice is tough, but it's not going to stop a bullet, not even a little .22, it will just shatter. In fact, you could probably hit it with a high-power BB gun and make a fair-sized dent, referencing your diamond, edge-on paper.
A rather poor analogy is this: Kevlar jackets can stop bullets can keep you alive. But just a single layer of Kevlar fabric on your body will do absolutely nothing useful to stop you from being killed.
Diamond is hard... but also brittle.
great thinking. i don't know. because who would be dumb enough to do that just to acquire knowledge?
but diamond is the hardest known mineral on earth. but if a 50 cal. shot on a diamond with that size i think the diamond will still be broken.