Which minerals fade in light
Realgar, amethyst, green fluorite and the ruby silvers change irreversibly on a lit shelf. Which species are at risk, and why colour centres are the tell.
Realgar is the benchmark case: hardness 1.5, and daylight converts it irreversibly to pararealgar, an orange powder that takes the crystal with it. The arsenic sulphides are the most light-sensitive minerals a collector will own. Amethyst and green fluorite fade in strong light, and the ruby silvers darken.
In short
- Two mechanisms, not one. A photochemical reaction destroys the species outright, as when realgar becomes pararealgar. Bleaching of a colour centre leaves the species intact and takes the colour away, as in amethyst.
- The published conservation guidance names the group explicitly: the arsenic sulphides are the most susceptible minerals to light damage, and amethyst and green fluorite fade in strong light.
- Neither is reversible on a shelf. Faded amethyst can in principle be re-coloured by irradiation, but that is a treatment, not a repair, and it is one that must be disclosed if the specimen is ever sold on.
- The dose is cumulative. A window that gets two hours of low winter sun does more damage over ten years than a bright display cabinet does in a week, and it does it to the same face every time.
- Storage is the whole answer and it is nearly free: an opaque individual box, and open the drawer rather than the curtains.
| Species or group | What light does | Reversible? | How to store it |
|---|---|---|---|
| Realgar, orpiment and the arsenic sulphides | Realgar converts to pararealgar, a powdery orange breakdown product; the crystal disintegrates with it | No | Light-proof box, opened briefly and rarely. The most serious case on this list |
| Proustite and pyrargyrite (the ruby silvers) | Long reported to darken and lose transparency at the surface on exposure | No | Dark storage as standard trade practice |
| Amethyst | Purple fades toward pale grey or colourless in strong light | Only by re-irradiation, which is a disclosable treatment | Out of direct sun; a closed drawer is enough |
| Green and some purple fluorite | Fades in strong light | Not practically | Out of direct sun. Beware south-facing windowsills |
| Smoky quartz, some blue topaz, some blue and green fluorite | Colour is a radiation-induced colour centre, which light and heat both bleach | Only by re-irradiation | Treat as amethyst. Never leave in a hot car |
| Vivianite | Darkens from near-colourless through blue to black; the change is an oxidation and dark storage is the conventional precaution | No | Sealed, dark, and handled as little as possible |
| Silver, copper, bornite, chalcopyrite | Surface tarnish. This is sulphidation from the air, not a light effect | Partly, by cleaning | Sealed box; darkness will not help and is not the issue |
The two-question light test
You do not need a memorised list. Almost every light-sensitive mineral a collector will meet is caught by two questions, and this is the rule worth carrying:
One: is it an arsenic or antimony sulphide, or a silver sulphosalt? If yes, it goes in the dark, full stop. This group contains the species where light does not merely change the colour, it changes the mineral. Realgar is the type case — and it is the one species on this page where the damage is genuinely fast, in months rather than decades.
Two: is the colour a colour centre rather than a chemical colour? A chemical colour comes from an element that is part of the structure: the green of dioptase is copper, the red of erythrite is cobalt, and neither is going anywhere. A colour centre is a defect in the lattice, usually produced by natural irradiation over geological time, and light and heat both undo it. Amethyst, smoky quartz, most blue topaz and much blue and green fluorite are colour-centre colours. That is the whole reason they fade and dioptase does not.
If either answer is yes, the specimen belongs in an opaque box. If both are no, ordinary sensible storage — out of the sun, stable, individually boxed — is enough, which is the standard we set out in handling and storage.
Realgar, and why it is in a class of its own
Realgar, As4S4, is soft — hardness 1.5 to 2 — and a fine crystallised specimen is one of the most striking red minerals there is. It is also the clearest case of light destroying a species rather than merely bleaching it. Exposure converts it to pararealgar, a friable orange-yellow powder with the same chemistry and a different structure. The volume change breaks the crystal up as it goes, so what you are left with is not a paler realgar but a heap of powder holding the shape of one.
Two practical consequences. First, buy realgar already boxed, and be suspicious of a dealer's display piece that has been under lights at a show for three days. Second, the powder is a friable arsenic sulphide, so the handling rules on our toxic minerals page apply with more force to a degraded realgar than to a sound one: damp-wipe, never dry-brush.
Orpiment, the other common arsenic sulphide, is more stable than realgar but sits in the same group and takes the same precaution. So does the classic association of the two together on a single matrix, which is the form most collectors own.
Amethyst and fluorite: the fade you will actually experience
Realgar is the dramatic case; fluorite and amethyst are the ones that quietly cost people specimens. Both fade slowly enough that no single day is noticeable and steadily enough that ten years on a lit shelf is obvious — particularly when a piece comes out of the cabinet and the face that was against the back is a different colour from the face that was toward the room.
Green fluorite is specifically named in the conservation guidance, and it matters here because it is a colour that British collecting is built on. Weardale material is the reason many collections exist at all, and its daylight fluorescence — the blue sheen over the green — is exactly the interaction with light that makes people want to display it, which is unhelpful. The compromise most collectors settle on is honest: display the piece properly, in a cabinet, away from the window, and accept that a specimen kept for looking at is being spent slowly. A piece you are keeping as a reference or an investment goes in the drawer.
Amethyst does the same thing more visibly because the starting colour is stronger. Brazilian material in particular is reported to bleach quickly in direct sun. Note that heat does it too, which is the entire basis of the citrine trade: most commercial citrine is heated amethyst, a fact worth knowing when you are reading a listing, and one we return to on how to tell if a crystal is fake or treated.
What good storage actually looks like
The equipment list is short and unglamorous, and none of it needs to come from a dealer:
- Opaque individual boxes for the sensitive species. Card trays inside a closed drawer are sufficient; a clear lidded box inside a closed drawer is fine too, because the drawer is the light barrier.
- Position over intensity. Moving a cabinet off a south wall does more than any filter.
- Museum-grade UV film on the glass if a display cabinet has to be near a window. It removes the most energetic part of the spectrum, which is the part doing most of the work, but it does not make a light-sensitive species safe on display — it buys time.
- A note on the label. Write light sensitive on the box. You will remember; whoever inherits the collection will not.
Do not do the two things that feel protective and are not. Sealing a specimen in a dark airtight container without thinking about humidity trades one problem for another — that is how pyrite decay gets started. And do not lacquer anything: it neither blocks light usefully nor is it removable, and it is an alteration that has to be disclosed.
Buying, and what to ask
Fading is a condition question, and condition questions are the ones that are awkward to ask and always worth asking. Three that get useful answers:
- How has it been stored? A seller who can tell you the specimen has been in a closed drawer since it came out of an old collection is telling you something real. A specimen photographed on a windowsill is telling you something too.
- Is this colour original? For fluorite, amethyst and topaz this is a treatment question as much as a storage one. Irradiation to restore or improve colour is a disclosable alteration.
- Which face was toward the light? On old collection material, uneven colour between the front and back of a single crystal is the fingerprint of a display cabinet, and it is a fair basis for a conversation about price.
None of that requires a specialist. It requires the specimen in your hand under decent light, and a seller who answers questions in writing. If you would rather have somebody else ask them — and be told the answers before the piece is committed — that is what the wanted list is for; we hold no stock, so the only thing we can offer on a light-sensitive species is having looked at it properly first.