Gem of Harbinger's Acid Barrier
From Drunkapedia
Stackable: Yes, 100(?)
Obtained By:
One of the possible rewards of the Aerbax's Prodigal Harbinger quest.
Give the Blood of the Harbinger to Issk (instead of the Emissary )and you recieve a random buffing gem for a certain element (light, fire, frost, or acid)