Dear Pat and Dennie,
I can’t advise you about Oxalic Acid, since I’ve no experience with
it, but I can recommend an alternative. A longtime rockhound and
mineral dealer, Steve Solovieff of Freeform Creations, once told me
that nothing cleans crystals better than Easy-Off Oven Cleaner (the
fume-free type is OK).
Just put the crystals in a sealable container, spray generously,
cover, and leave overnight. I have used this method with white
chalcedony roses and delicate kammererite drusy with great results.
Give it a try.