[Rockhounds] Cleaning quartz and geodes

[Joshua Stiff]

I have been cleaning Lolo smoky quartz specimens and Challis volcanic
agates/geodes using muriatic acid.  The results have been mixed.
Sometimes the specimens come out beautifully, while other times I end
up with a yellow staining.  I've tried re-bathing the specimens in
muriatic, and again, sometimes this helps, sometimes it doesn't.  Its
really bizarre to me as I can't pin-point any concrete similarities as
to why the staining occurs.  My best guess is that depending on what's
reacting in the load I am washing, perhaps the temperature in the
specimen's matrix increases enough with the reaction to allow acid to
permeate the specimen?  I would assume then, when the acidic reaction
stops, HCl is trapped inside the specimen causing yellow staining?  I
am no chemist so its a guess :)

Anyways, advice would be helpful.  I've googled the topic multiple
times and have found many "becareful when working with acid topics".
I understand that this probably means I should be a chemist before
playing with acid :)  However, considering that I am using the HCl
with the proper safety equipment (as directed from many different
sources), it would be nice to find more advice on how to properly
clean quartz minerals with HCl.  I've tried 27% HCl at 100%, and a
50:50 mix with distilled water.  I also have my baking-soda/distilled
water rinse mix ready.  Still, the yellow staining has occurred at
least once in every batch I've tried.

Sincerely,
slightly frustrated