I have gone over the patterns I have for such a belt; it does require and emerald, which must be enchanted, and an diamond enchanted with a spell of grace. The actual leather is dire bear, which is treated with some alchemical essences of grace.
I once saw such a belt, it seemed to increase the ability to move quickly, along with strengthening mental resolve.
Given the nature of the risk involve, you may wish to seek a better tailor than myself. With my normal equipment, I would guess I am as likely as not to be successful, although my chances would be much greater with some spells and loaned equipment. If you do wish to contract with me to make the belt, we can discuss a price, and what supplies I would need provided.
~Drexia
//takes 2 dire bear leather
1 enchanted emerald
1 diamond enchanted with cat's grace,
10 essence of grace
pattern and thread.
+1 AC, +2 dex, +6 concentration, +4 spellcraft.
Edit: I was just told that the stats changed, and the belt does not have an AC bonus, but has +1 Int and +1 Cha
I have a 50% chance without additional help.