Why is my amethyst losing color
Amethyst is quartz, hardness 7, a crystal coloured by iron that radiation altered. Strong light fades it; 400 to 500 °C turns it brown. Or it is not amethyst.
Why is my amethyst losing color? Strong light or heat, or it was never natural amethyst. Amethyst is quartz, hardness 7, purple because radiation altered iron in the crystal. The NatSCA conservation leaflet says purple amethyst fades in strong light; the International Gem Society says 400 to 500 °C can turn it brown, red or green.
In short
- Amethyst is quartz, and its colour is a defect, not an ingredient. Cohen, in American Mineralogist in 1985, attributes the colour to iron in the quartz altered by ionizing radiation. The quartz sheet itself never uses the word amethyst; it lists bluish violet among colours from chemical or particulate inclusions. None of the 176 sheets in this site's reference set uses the word.
- Light fades it. The NatSCA leaflet Care and Conservation of Geological Specimens names purple amethyst among minerals that will fade in strong light. It gives no timescale, and neither does this page.
- Heat turns it brown, red, yellow or green. The International Gem Society says amethyst heated to 400 to 500 °C may turn brown, red and sometimes green, and that amethyst heated to yellow or red-orange is by definition citrine. Cohen heat-bleached amethyst at 500 °C, and X-irradiation then brought the original spectrum back, with additional absorption centres.
- Turning white or black is usually something else. A white film is a deposit, and a smoky tone is, on Cohen's account, a different, aluminium-related colour centre. No verified source for this page describes a household process that blackens amethyst.
- Or it was never amethyst. Twenty of the 176 sheets record a violet-to-purple shade, and three of those are hardness 7 or more throughout: corundum, quartz and topaz. Purple fluorite, hardness 4, is the common confusion, and the 2.9 line separates them: weigh a 20.00 g piece in water, and quartz reads 12.45 g, fluorite 13.70 to 13.72 g.
| What you see | Likely cause | The number behind it | Test that separates it | What to do next |
|---|---|---|---|---|
| Paler overall, most on the side that faced a window or lamp | Light fading of the colour centre | NatSCA: purple amethyst fades in strong light; no rate published there | Compare the lit side with a side that was in shadow | Out of strong light; the storage rules are on the fading page |
| Brown, yellow, orange or green after heat | Heat destroys the colour centre | IGS: 400 to 500 °C; Cohen bleached at 500 °C | The change is through the stone, not one face; a heat source in its history | Not reversible on a shelf |
| Purple and yellow zones, or patchy colour, from the start | Natural zoning | Quartz sheet: zoned or mottled | The pattern matches old photographs or the label | Nothing; that is the crystal |
| Colour strongest in cracks, or a streaky, uneven surface colour | Dye on quartz | Quartz sheet: the base colour is colourless or white | Colour concentrated along cracks under a 10x lens | The dyed-crystal checks |
| A white film or white patches | A deposit on the surface, not lost colour (our judgement) | Documented changes: paler in light; brown, red, yellow-orange, green or blue with heat; none is white | A 10x lens shows a layer on top of the purple | The white-crust and cleaning pages |
| Dark, smoky or black zones | A separate colour centre, or a coating | Cohen: smoky colour comes from aluminium-related centres | Edge-on: through the crystal or only on the surface | Nothing, if it is in the crystal |
| Softer than expected, heavier in the hand | Not quartz: purple fluorite | Hardness 4 against 7; the 2.9 density line | A 20.00 g piece reads 12.45 g in water as quartz, 13.70 to 13.72 g as fluorite | The real-fluorite page |
Why is my amethyst losing color? What makes it purple in the first place
Amethyst's purple is a radiation-induced colour centre involving iron, not a pigment, and that is why it can be lost. Cohen, in American Mineralogist 70 (1985), explains that interstitial ferric iron in quartz is oxidised further under ionizing radiation and that the amethyst colour is due to an absorption band related to the iron so produced. The same paper notes that the ferric iron needed predominates in quartz grown near the surface, usually in cavities or geodes, which limits where amethyst forms. The International Gem Society puts it more simply: iron and other trace elements, plus natural radiation.
Method. The 176 sheets in this site's reference set were converted with pdftotext -layout and each Color field was extracted by script. No sheet uses the word amethyst, the Handbook of Mineralogy sheet for quartz included; its colour line lists bluish violet among colours from chemical or particulate inclusions. Twenty sheets record a violet-to-purple shade (violet, purple, lilac, lavender or mauve); three of them are hardness 7 or more across their whole range: corundum, quartz and topaz. Quartz is hardness 7 with a measured density of 2.65.
So the diagnostic question is always: what has destroyed or hidden the colour centre, or was it ever there? Light and heat destroy it; a surface deposit hides it; dye, glass or another mineral means it was never there.
Can sunlight fade amethyst? What the conservation guidance says
Yes: the NatSCA leaflet on geological specimens names purple amethyst, with green fluorite, among minerals that will fade in strong light, and recommends light-proof containers for susceptible species. It gives no exposure time or threshold, and no measured fading rate was found for this page, so none is quoted.
How to see it, in our judgement: fading from light should follow the light, so a crystal that stood in one place may be paler on the side toward the window than on the side against the wall or the base. The broader question of which species fade, with storage precautions, is on which minerals fade in light, and display lighting on lighting a mineral display cabinet; neither is repeated here, and neither has been tested on amethyst for this page.
Purple fluorite, the commonest look-alike, is not what the leaflet names: it names green fluorite beside amethyst. Fluorite as a species is on the fluorite page, and telling the two apart is in the last section below.
Why did my amethyst turn brown or yellow? Heat, and the 400 to 500 °C figure
Heat removes amethyst's colour centre: the International Gem Society says amethyst heated to 400 to 500 °C may turn brown, red and sometimes green, and that amethyst heated to yellow or red-orange is, by definition, citrine. It also notes that heat treatment can lighten amethyst, and that names such as Madeira topaz for heated amethyst are misnomers, topaz being a separate species.
The laboratory version: Cohen's paper shows a Brazilian amethyst heat-bleached at 500 °C, then X-rayed until the absorption spectrum stopped changing; the original spectrum returned, along with additional absorption centres the natural stone had not shown. Heat takes away the colour centre, not the iron, which is why irradiation can restore it. Re-irradiation is a treatment, and treatments are a disclosure matter, covered on the fading page.
Every one of those temperatures is below the one on the quartz sheet. The sheet's polymorphism line gives 573 °C as the limit of quartz's stability, so amethyst loses its colour below the temperature at which the quartz itself changes. This page does not say what temperature a car, lamp or windowsill reaches; if a stone turned brown or yellow after heat, the history is the evidence.
Is my amethyst turning white or black, or was it never amethyst?
White patches or a white film are, in our judgement, usually something on the stone rather than lost colour, because the documented changes, fading in strong light and brown, red, yellow-orange, green or blue after heat, do not include opaque white. See why is my specimen growing a white crust and how to clean mineral specimens. A dark or smoky tone is a different centre on Cohen's account, the aluminium-related one that makes smoky quartz, and a black surface film is dirt until shown otherwise.
The 2.9 line, stated once: a purple stone denser than 2.9 is not quartz. Quartz measures 2.65; fluorite 3.175 to 3.184, higher if rich in rare earths. Weighed in water, a 20.00 g quartz piece reads 12.45 g and a 20.00 g fluorite 13.70 to 13.72 g, calculated as mass times one minus one over density. The method is on how to measure specific gravity at home, and the fluorite side on how to tell if fluorite is real. Fluorite is also hardness 4 with perfect {111} cleavage, so it breaks along flat planes where quartz, with poor cleavage, breaks conchoidally.
Dye and glass are the other never-was cases, on how to tell if a crystal is dyed and how to tell if a crystal is fake. The damage and identification pages are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.