From Chap 26
The key to a puzzle is often something you read twenty years ago in an old scroll, or a peculiar ring you saw on the finger of a man you met only once.
Possibly Voldemort made the Resurrection stone/ring into a Horcrux while killing his 'uncle'.
Later in the same chapter, Quirrell suggests that he's made Bacon's diary into a Horcrux.
"Oh," said Professor Quirrell, "don't worry about a little rough handling. You could toss this diary in a fireplace and it would emerge unscathed. In any case, I await your decision."
In canon, one of the main properties of a Horcrux is that it is indestructible by ordinary means, but since Quirrell has only 'recently acquired' this, it might just be charmed? Or would it be a Horcrux of Quirrell, rather than of Voldemort? Not sure...
[Edit] Original comment was screwed up, I had something in mind, and wrote parts of it here which made little sense
Just because Quirrell says that he recently stole it does not mean it is true. Telling harry that it is stolen property is a good way to make sure he keeps it secret without causing any suspicion about the nature of the book. I think that the diary is a horcrux and another attempt to turn Harry over to his dark side permanently.
(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: