The first time I set a white drusy I got rough embedded in all the
crystals when I did the final buffing. This was a particularly
rough surface drusy and the way I finally got it clean again was
soaking it in warm oxalic acid and then using hot Mr. Clean solution
and a soft brush.
Now if the piece needs any more buffing after it has been set I coat
the stone with clear nail polish first!
You might also try soaking the stone with some acetone and then
brushing it gently. I have never heard of one changing color but
I've seen some that were really dirty from wear like the grime that
gets under a stone in a ring or earring.
Bernardine Fine Art Jewelry