For the record, it never occurred to me it could be Riddle's diary, and yet I assumed it was a horcrux because it was indestructible. I was probably pattern-matching with the Pioneer probe, which Quirrelmort also said he had spelled to be indestructible and this was, indeed, a Clue that it was a horcrux.

Also, in canon, a horcrux can possess you if you become emotionally attached to it. The diary seemed calculated to become a prized possession of Harry's, and that when Harry learned Latin he could well be in for a nasty shock...

Perhaps I should have updated when it was never mentioned again. Hmm.