Dropped off a Servant of the Magistrate . . . rare drop here it seems. Only have seen the one so far.
Rune of Coalition and Rune of Conjuction are relatively 'common' drops on the various mobs across GE, including the servant, A Tormented Mercenary, and A Shrouded Willsapper.
*** Edit *** 11 days later, on 1/1/05 after multiple kills, another Rune of Divergency drops, so definitely uncommon in GE.