Something I thought about recently, but first some data:

What does the Fidelius do?

Well, if you put a house under the Fidelius, it becomes impossible to find unless you know the secret.

This must mean that people who already knew about the house, forget it ever existed.

Aditionally, they never question that they forgot about it. Bellatrix Lestrange did not appear to remember Grimmauld Place, despite being raised there.

It also seems to me that not only memories are affected, but all pictures, writings and recordings of the object/house under the Fidelius are modified by the spell.

And now you know why it is a terrible idea to put the Philosopher's Stone under the Fidelius.

Think about what would happen if Dumbledore put the Stone under the Fidelius:

1. Flamel would forget that it exists and that he created it.

2. Everyone else would forget that the Stone ever existed.

3. Flamel's fame would be erased, since he got it thanks to creating the Stone.

4. Dumbledore would be left with a tempting choice to simply not tell Flamel just what they have hidden under the Fidelius. He would have the Stone for himself. Granted, Albus isn't like that, but the temptation would be there. Would Flamel really trust Albus like that?

5. Flamel would not remember that the Elixir of Life can be made and he would die.

6. If this is done after the Stone was removed from Gringotts, Voldemort and Quirell would forget the reason they are at Hogwarts. Naturally, they'd turn to Harry and perhaps would even succeed in killing him.


These are my canon-support reasons, feel free to refute them.

 

Fidelius doesn't mean that you can't know something exists, it just means that you can't see or touch it. The Death Eaters in DH were able to wait outside the front of Grimmauld Place, they knew that it existed and where it was they just weren't able to see it or get at it.

 

Oops, completely forgot that. Still doesn't fully explain Bellatrix.

 

Bellatrix remembered Grimmauld Place. She just didn't remember it as Grimmauld Place. She remembered it as that one place she grew up in whose name and exact location she couldn't quite recall.

 

Can you quote that? I'm quite interested in the exact phrase.

 

Er, why would Bellatrix have grown up at Grimmauld Place?

Or were all the branches of the Black family living in the same house because they couldn't afford to do otherwise? Boy, that would put a real dent in their fanon image as ancient, noble, and filthy rich.

 

Besides all of the above, Dumbledore wouldn't be casting the Fidelius on the idea of a philosopher's stone, or even on this particular philosopher's stone. He would presumably be casting the Fidelius on the location in which the philosopher's stone was hidden.

People don't forget about the existence of Harry Potter the moment he steps inside Grimmauld Place.

 

The way the fidelius charm works is never properly defined in canon - what does it mean to hide something, in the context of preventing other people from knowing information about it?

 

I think it's defined by result rather than by any particular methodology. Whatever method a person may use to try to discover a place under the Fidelius, they can't unless they're told the secret.

 

So it hides a physical location? Ok, at least that's self-consistent. The idea of hiding ideas makes me want to pull my hair out.

 

Weren't we led to believe that there was only one stone?

 

Headcanon hat on:

It seems that there must be a limitation to the Fidelius such that the more people who know about the secret before the spell is cast, the harder the spell is to cast.

Otherwise you could cast it on public locations ("The Ministry of Magic") or huge areas like entire nations.

So if you can cast the spell on ideas -- which I'm not opposed to, as I feel that spells are generally quite flexible like that -- then it would only work on "small ideas". You couldn't cast a Fidelius on the idea of money, for example. While theoretically it could be possible, no wizard could pull it off. But maybe if it was a small secret - like a family secret, or a new discovery - you could do it.

Edit:

There is only one existing at this current time. That doesn't mean that it's a rule of the universe that there can only be one. So even if you cast a Fidelius on this stone, rather than its location, it doesn't mean people suddenly can't access the idea of philosopher's stones. Just Nicolas Flamel's stone.

 

Harry who?

 

Even if Harry could be afforded such protections, they might have backfired. A Voldemort no longer preoccupied with the murder of an adolescent boy might have taken the world with ease.

 

Does that mean if someone murders somebody, but then hides the secret that he was the murderer, he cannot be caught?

This would be a small secret considering that only he knows of it initially.