Actually it's a property of the "head" area on the character info that any item put there would protect against a critical hit, so Ioun Stones protect as well as the mask

I have an Ioun stone on both Imoen and Milo. Milo, as you probably aren't aware of, is a Kensai

Ioun Stones do protect from critical hits.

Mages/Sorcerers can wear Ioun Stones, but not all of them. The best Ioun Stone a Mage/Sorcerer can wear in SoA is the Pale Green Ioun Stone. There is a better one in ToB.

mages CANNOT wear ioun stones in the original version of SoA
If you have ToB installed however they CAN wear Ioun stones (which seems more logical to me)

<blockquote>quote:</font><hr>Originally posted by Faceman:
mages CANNOT wear ioun stones in the original version of SoA
If you have ToB installed however they CAN wear Ioun stones (which seems more logical to me)<hr></blockquote>

Incorrect, and I repeat my previous statement.

Mages/Sorcerers can wear Ioun Stones, but not all of them.