I finally got one of these. It's probably just a coincidence, but I got it in a fight where I avoided all of the damage from its "volley of stones" attack. Maybe if the stones lodge in your body they're not good for collecting?
I was not Etheric. I did have a Myers sampling kit equipped. I did not have any mining-related gear equipped other than the scuffed mining helmet.
EDIT: Slipped past all the volley of stones damage again and didn't get a drop, so if there's anything at all to this theory, it's not the whole story.