Light fading

The failure that happens in the display case, in front of you, too slowly to notice.

Color is not always permanent

A number of minerals owe their color to structural defects or to trace elements in a particular oxidation state, and light supplies the energy to undo that.

The change is gradual, irreversible and invisible day to day. You notice when you open a drawer of the same material that has been in the dark.

Which is the diagnostic worth knowing: keep one specimen of a fugitive species in the dark as a control.

The known fugitives

Realgar, which not only fades but degrades to pararealgar and crumbles. It is the classic and it is fast.

Amethyst and smoky quartz, whose color comes from irradiation-induced centers that light and heat undo.

Rose quartz, reliably.

Celestite, fluorite and vivianite, variably.

Proustite and pyrargyrite, which darken.

Irradiated and heat-treated material is frequently among the least stable, which is one reason treatment disclosure matters to a buyer.

How much light it takes

Less than people expect. Daylight through a window is intense, and ultraviolet is the most damaging component.

It is cumulative. Hours add up, and a specimen on a sunny shelf for five years has had a large dose.

Heat comes with light from some sources and compounds the problem.

Displaying anyway

A collection nobody looks at has a different problem, so this is a trade rather than a prohibition.

Keep fugitive species out of display, or display them briefly and return them.

Away from windows, and UV-filtered glazing on cases.

LED rather than halogen, for heat as much as for spectrum, and lights off when the room is empty.

And flag the fugitives in the catalog, so the decision is made once rather than every time the case is rearranged.

References