spectre:
So I really need ideas about how to present the holocron-thingy in a way that seems both mundane and potentially incredible.
Any help?
Your best help is going to be from your players themselves. Anything you come up with yourself or that we help you come up with might have the desired effect on you, or us, but is likely to fall flat with anyone else, including your players.
That said, the classic approach is to present something that seems normal, like a wooden box, but that seems just a bit off. Like, when you rotate it to look at it, the designs carved into it change. Like, if you were looking at a carved triangle, and you turn it to look at the symbol on another face, but then turn it back, the symbol is not the triangle but something else.
The problem with something like that, though, is that the players might think there's a point to that and try to test it out, with cameras or whatever. But that's not the point, that's just part of the incredibleness of it.
Depending on your players, this problem might arise no matter what you do, even if you go with the Lovecraft classic of the angles being non-Euclidian. They'll have their characters break out protractors and try to figure out that "mystery" when there really isn't one.
So, when you go to your players for suggestions, make sure they understand that the suggestions don't have to do anything practical or have anything to do with how the item functions, but just need to be what they think of as creating that sense of incredibleness.
Good luck.