Display cases
Updated
A collection nobody sees has a different problem from one that fades, so this is a trade rather than a prohibition.
Display against conservation
Everything that makes a specimen visible, light, an open case, a warm room, a wall people pass, is something the conservation pages argue against.
The workable position is a rotating display: a small number of specimens out at a time, chosen from the ones that tolerate it, changed regularly.
That also makes the collection more interesting to visitors than a static wall.
Light
UV-filtered glazing on the case.
LED lighting, for heat as much as spectrum, positioned outside the case where possible.
Away from windows. Daylight is far more intense than people judge it to be, and it is cumulative.
Lights off when nobody is in the room, which is the cheapest control there is.
Heat
An enclosed case with lighting inside it is an oven at a small scale, and every reaction in the what-goes-wrong section runs faster warm.
It also cycles: warm while lit, cool overnight, which moves humidity in and out of the specimens.
Ventilate the case, or light it from outside.
What to put in one
Stable species: quartz other than amethyst and smoky, feldspars, garnets, most silicates, native metals.
Not the fugitives: realgar, amethyst, rose quartz, celestite, vivianite, proustite.
Not actively decaying sulfides, which need a sealed dry box, not a display.
And not the best specimen in the collection, unless you have accepted what a display costs it.