Well, that's funny... My son did not find it either, but he did find a baldurian helmet I did not find. Could some objects be different from one game to another ? The only difference is that I imported my fighter from BG 1, while he created his fighter-mage.
By the way, here are the characteristics (sorry, I have the french version, so some words may not be correctly translated) :
When activated, it gives you for some time :
Immunity against 1st and 2nd level spells
Save against death, wands, metamorphosis, (wind ? I don't know what it is) and spells : + 2
AC +5 vs missile weapons
Deals some unknown damages when activated.
Cannot be used by clerics, druids and thieves.
Voila... Hope this helps.