Recently in my message boards the question came up of how hard is it to attack or grab someone else's ioun stone. The DMG states that they are AC 24, but that doesn't give us quite enough info. So, here's my take on it.
Attacking an Ioun Stone
A moving ioun stone is an attended object, so I'd treat it as a "held, carried, or worn object" as defined on page 135 of the Player's Handbook. The relevant text from the D20 System Resource Document is:
"An inanimate, immobile object has an AC of: 10 - 5 for no Dexterity + its size modifier."
Ioun stones are Diminutive, so they get a +4 size bonus to AC. But they're not immoble, they're held (technically), so use the rules for held objects:
"Attacking a held, carried, or worn object provokes an attack of opportunity. Objects that are held, carried, or worn by a character, are harder to hit. The object uses the character's Dexterity modifier (not its own -5) and any magic deflection bonus to AC the character may have. The attacker doesn't get any special bonus for attacking the object. If the target object is in the opponent's hand, it gets a +5 AC bonus because the opponent can move it quickly out of harm's way. "
So it uses the bearer's AC instead of the default -5 (for "0 Dexterity," not the same as "no Dexterity," a terminology goof).
While an ioun stone is technically not in the bearer's hand, it's more mobile than, say, a belt pouch, so I'd treat it as if in the bearer's hand. That's another +5 to its AC.
So it's AC is
10 + 4 (Diminutive size) + 5 (in-hand) + bearer's Dex modifier + bearer's deflection bonus.
Attacking it provokes an attack of opportunity.
Grabbing an Ioun Stone
To grab the stone, instead of striking it with a weapon you just try to start a grapple with it. You have to have a free hand (no weapon in it) and make a touch attack with that hand. If the touch attack succeeds, you can make an opposed grapple check against the ioun stone.
For the opposed grapple check, I'd treat the stone as 10 Str (while the object itself cannot lift weight as if it had 10 Str, it should get some benefit for being an attended item).
Neither side should get a size modifier (you're just using your hand, so your whole body size shouldn't count).
If you succeed at the grapple check, you've grabbed the stone and can move it away from the opponent (you don't need to make another check to establish a hold, we're just talking about a rock!), which means it is in your possession.
If in the attack of opportunity your opponent chooses to grapple, he
can either grapple you directly while you try to grab the ioun stone,
or he can grab at the stone itself (no roll needed because it's
his own item and you haven't grabbed it yet) and force you to attempt to
grapple him instead of the stone. The former means that you may
end up holding the stone but the opponent may end up holding you.
The latter means that the two of your are making opposed grapple checks
for control of the stone; in this case, your opposed grapple checks
are against the opponent's Strength (rather than the default 10 Str for
the ioun stone), and you have to get a hold (rather than just a
grab) to wrest the stone away from your opponent.