Obtained this from 'a goblin stonechanter' in Nurga on 6/19/03. I was working my way from one end of Nurga to the other, killing every spawn on the way in hopes of getting a rockchanter to pop. I never saw one, but the last stonechanter I killed dropped the slate rock. I've probably killed a couple dozen stonechanters and at that point I had only seen the blue & green rocks, with the blue one being the most common. Was totally shocked to see it on a regular stonechanter, considering that every post on the subject says it only drops on the rockchanters (I think they were removed).
Good luck getting one!
I'll add a screen shot later.........
Edited, Thu Jun 19 15:46:10 2003