Well, Maury, sounds like with all your excellent experimentation you are discovering the answers to your own questions! Good job!
That has been my experience at removing the “gunk”. Some of it will flick off with an x-acto knife and have a beautiful un-etched crystal surface beneath and some will take 3 or 4 or more soakings in acid or Iron Out, to remove satisfactorily, only to be left with an etched surface, and some will be partially included in the quartz and be permanent.
There are different types of “gunks” depending on the locality and mineral being collected, and it can be a decision whether to try and remove it or not. This coating from Maury Mtn looks like a secondary or tertiary coating of silica, or quartz, possibly mixed with feldspar, mica, and other mineral detritus that settled over time onto the crystals, which is why you may have gotten and initial reaction to the acid. But Scott or someone may have a better explanation.
I'm still getting a kick out of you choosing your namesake mountain location to collect at,
, and I've done a little surfing to see what's being found out there today. I did see some pieces like what you found and yes, they are still finding the golden brown, yellow, green and red moss, too. Oregon sure makes some pretty agate!
Good luck and good job!