Oh, yeah. You know, that's actually one of the poll questions on JKR's site - whether that's a pensieve or not. Hmm... Well, for sure, it's not Dumbledore's pensieve. This is obviously not a portable one, at least. If it is a pensieve, maybe it belonged to someone from the past and it contains their memories? Maybe it's Gryffindor's pensieve? And maybe he put his memories in it before he died or something? Hmm... Or maybe it's not really a pensieve but something similar. I'm thinking maybe sort of like the archive section of a library, something. And it contains images of historical facts (sort of like a video archive or whatever). I don't know!