Lately I learned some valuable information: orpiment degrades in sunlight. One should keep orpiment (and realgar) samples locked up in a dark place.
Orpiment, for those who don't know, is an orange aresenic mineral, often associated with its red counterpart, realgar, and often associated with calcite at certain types of gold deposits.
I collected some calcite-orpiment-realgar samples while at Carlin mine some years ago. I noticed that the realgar had more or less disappeared before I moved to California, but the orpiment was still a vibrant pretty orange, and I was happy enough by the colour that I didn't mind that I was carting around, and decorating my apartment with, toxic arsenic minerals.
For the past year plus I have been living in sunny sunny California with rocks lining all my windowsills. Sure enough the orpiment samples are included in this generality.
If we add the first statement with this last statement, I think you can figure out why I need to get cleaning and (sadly) toss these samples.