Light
Direct sunlight is the single biggest collection killer. UV bleaches fluorite (a deep purple specimen can go to colorless in a few months on a sunny shelf), darkens hackmanite, fades realgar to pararealgar, and yellows vivianite. Use UV-filtering display glass or keep the cabinet out of direct sun. LED lights with low UV output are safe; halogen and unfiltered sunlight are not.
Humidity and temperature
Pyrite, marcasite, and other iron sulfides oxidize to acidic sulfates above ~60% RH — first as a white crust, eventually destroying the specimen and any neighbors. Keep these minerals at 40–50% RH in a sealed cabinet with desiccant. Water-soluble minerals (halite, borax) require even lower humidity. Sudden temperature swings crack large quartz crystals and brittle prisms — keep the room reasonably stable.
Handling, mounting, and labels
Touch as little as possible — skin oils etch soft minerals and stain water-soluble surfaces. Use cotton gloves for valuable pieces. Mount large or unstable specimens on display bases with mineral tack (a removable putty), not permanent adhesives. Every specimen needs a catalog number (a small inked dot + number) AND a paper label inside the box with locality, mineral, dimensions, and acquisition date. A collection without provenance loses half its value at every transfer.