Captain America: Let's start with that stick of his. It may be magical but it works an awful lot like a HYDRA weapon.
Nick Fury: I don't know about that, but it is powered by the cube.
Bruce Banner says it emits the same gamma signature as the Tesseract.
Can't think of any more.
I think fury is making the conclusion based on his observation that it is emitting the same gamma signature as the Tesseract. From the scientific method's stand point (yes, I just started school this week -_-) all it is, is a guess, a hypothesis based on his observation. If all we have to back up that it is powered by the Cube, is Nick Fury making that statement based off an observation, then sure, it could simply be an observation.
Humor me.
Now, while I don't believe it is the mind gem in the scepter, I am trying to see it from both sides. Who is to say the mind gem, or any of the gems for that matter, DONT emit the same radiation as the tesseract?
We don't know that they DON'T because the gems haven't officially been revealed in the MCU yet.
So with that being said, lets say the gems DO give off the same radiation as the Tesseract.
Based off what Fury has said, it can easily be guessed that, whatever that thing is in the scepter, is powered by the tesseract, when in fact, it isn't. It is just giving off the same radiation.
Now again, right now, I don't believe it is the mind gem.
However, based that dialogue, it seems like Fury draws that conclusion based on his observation, and that is just what it is, an observation, which lead to an assumption which hasn't been backed up.
Could I be reaching? Maybe, but the point is, there are points for either side.
So as of right now, anyone attacking someone of the other thoughts really shouldn't be doing so, because there really isn't anything to prove you are right, and they are wrong, since what has been said about the scepter is just based off an observation made by Fury