Minerals look permanent and several of them are not.
What actually destroys them
Light. A number of minerals fade on prolonged exposure, and the change is permanent. If you do not know whether yours is one of them, assume it is.
Humidity. Some minerals absorb water from the air and break down; others effloresce into powder. Again, if unsure, keep them dry.
Contact. Minerals are hard and brittle, so two specimens in a drawer chip each other, and the harder one wins.
Handling. Oils from skin mark porous and soft specimens.
What to do instead
- Each specimen in its own compartment or box, padded, nothing touching
- Out of direct light, especially anything coloured
- Dry — silica in the drawer, replaced when spent
- Labelled with species and locality, because an unlabelled specimen loses most of its value to a collector the day you forget
Water
Do not wash a mineral you cannot identify. Several are water-soluble, several have inclusions that react, and a rinse can destroy a specimen in seconds.
Displaying
A lit display case is a slow bleach. Use indirect light, rotate what is displayed, and keep the sensitive specimens stored.
What it costs
Compartment boxes, labels and silica. The labels are the part people skip and the part that matters most to value.
Questions people ask
How do I know if mine is light-sensitive?
Look up the species. If you cannot identify it, keep it dark.
Should I oil a specimen to bring out colour?
No. It is a treatment and it must be declared when selling.
Does a cracked specimen still sell?
Yes, described. Hiding it is what does not work.
More: describing minerals, common mistakes.