Price Varies; Aura moderate conjuration; CL 7th; Weight —
Minor 18,000 gp; Major 42,000 gp; Greater 66,000 gp
Alternating diamonds and rubies stud this band of white gold. A minor ring of fortitude reduces ability damage or temporary ability penalties the wearer takes by 2 points and ability drain by 1 point. If an effect targets multiple ability scores (for example, exhaustion, fatigue, or touch of idiocy), the ring reduces the damage, drain, or penalty for all ability scores.
A major ring of inner fortitude reduces ability damage or temporary penalties by 4 points and reduces ability drain by 2 points.
A greater ring of inner fortitude reduces ability damage or temporary penalties by 6 points and reduces ability drain by 3 points.
Wearing a ring of inner fortitude ring does not make the wearer immune to conditions such as fatigued or exhausted even if the wearer ignores all ability damage, drain, or penalties from the condition.
Minor 9,000 gp; Major 21,000 gp; Greater 33,000 gp
Forge Ring, restoration
According to the new errata, if the ring prevents a penalty/damage that is associated with gaining a benefit (such as a mutagen), then you don't get the benefit either.
I suggesting ignoring that errata. However, if the OP was mostly interested in the mutagen stat penalties, there is the 'Vest of Stable Mutation' which gets rid of the stat penalty from mutagens.
That erratum makes sense. The ring is plenty powerful without ignoring a balancing factor that they added, and I say this as someone who is losing out on a cognatogen on my psychic due to the ring.
*shrug* I disagree. Most of time you could compensate for the ability damage using things like boro beads and potions of lesser restoration, without taking up a ring slot, for a lot less money. The ring is a convenient, though expensive, way to avoid dealing with the hassle.
Also, at higher levels an alchemist is immune to poison, which is one of the primary causes of ability damage, so the ring in general is a lot less useful to you. That in and of itself more than compensates for the mutagen/cognatogen benefit.