ITT: Obvious shit that would revolutize the world, and yet people haven't thought of it yet.

Diamond body armor.

Scientifically it's impossible to broke a diamond unless you hit it with another diamond right? Why don't they make diamond sheets of armor, and then put it in between a vest or something so it's a diamond bullet proof vest.

The only way to kill the person would be to shoot them with diamond bullets, and since diamonds are hard to find, it wouldn't be cost effective to make diamond bullets for a whole army.