Balvan made his special stone breastplate some time after he was born. He probably tinkered with it afterwards.
It's a +4 Breastplate and gives an additional DR 4/- it also gives Acid resist of 4 and deducts 1 from every force effect (multiple times, like every missile from a Magic missile spell.) It also has double the armor check penalty and weighs 400 lbs. The wearer can also Track but only when touching bare stone and it only reveals paths across bare stone or in tunnels.