If we ever discovered a substance that we thought was harder than diamond, and tried to do the common "scratch test", would we even be able to? Even if it is harder, would it still be able to scratch diamond with just hand power? Or would we need some sort of machine.

I don't believe we are able to scratch diamond or granite now either.

I doubt scratching said substance by hand would work, from my rudimentary knowledge there would be a way to 'scratch' the supposed sample with another sample of the same stuff ala diamond cutter ...

Diamond can scratch granite, applied properly. I believe the answer lies in the way that diamonds are cut and shaped today... something has to do it, and I believe the "something" involves diamond-crusted tools.

You... just came up with this? Too much time on your hands?

Either use 'aggregated diamond nanorods(ADNR)' or 'ultrahard fullerite'.

I believe they let diamonds erode over long periods of time, that works even on diamond.

Diamonds are bery brittle in some directions. Diamonds are naturally octahedrons (or dodecahedrons, I forget which), and any sides that face in the same direction with the that basic form are extremely tough. Others are not. Also, diamonds are not entirely uniform in quality, and some parts just are weaker for no real reasons.

So, to shape a diamond you apply some diamond dust on it. Some edges in the dust are bound to be harder than some surfaces of the diamond you are working on.

I thought carbon atoms aligned in cubes to make diamonds so strong, but then again I had way too many pretty girls in my chem class.

And just curious, where does diamond dust come from, and not the obvious "from cutting diamonds", but the first diamond dust. How could they make it without having any, way back when.