Why is my selenite turning yellow
Selenite is gypsum, hardness 1.5 to 2, clear unless impurities colour it. On a specimen, yellow or black, flaking and chalky faces each have a cause and a test.
Why is my selenite turning yellow? Usually something on it, not in it. Selenite is gypsum, hardness 1.5 to 2, and its Handbook sheet puts every colour beyond colourless and white down to impurities, so a clear crystal that yellows on a shelf has, in our judgement, picked up dirt or oil. Flaking is cleavage; chalkiness is water or heat.
In short
- Selenite is gypsum. The International Gem Society calls colourless, transparent gypsum crystals selenite and the fibrous variety satin spar. The Handbook sheet gives CaSO4·2H2O, hardness 1.5 to 2, one perfect cleavage on {010} and a flexible, inelastic tenacity: a bent flake stays bent.
- The sheet records colour as an impurity effect. Its line reads Colorless, white; if colored by impurities, yellow, tan, blue, pink, brown, reddish brown, gray, black. Only 5 of the 176 sheets in this site's reference set name impurities or inclusions in their colour field. A colour a crystal arrived with is its own; a colour that appears on the shelf, in our judgement, is usually on the surface.
- The edge-on rule decides which: look through the crystal edge-on against white paper. Colour running through the thickness, zoned or as inclusions, is in the crystal. Colour confined to a skin at the faces, or strongest where fingers hold it, is on it.
- Chalky or frosted faces are water or heat, and both have numbers. The ILO-WHO card for gypsum gives a solubility of 0.24 g per 100 ml at 25 °C, and an apparent melting point of 100 to 150 °C that is really loss of crystal water. Driving all the water off would take 20.93% of the mass, calculated, which is the sheet's own H2O column.
- The order of causes on this page is our judgement of what is common, not a count. Flaking is cleavage, splinters on a satin spar wand are the sheet's splintery fracture, and an even, bright colour on satin spar may be dye.
| What you see | Likely cause | The number behind it | Test that separates it | What to do next |
|---|---|---|---|---|
| A yellow or brown tint that has grown, strongest where it is handled | Surface soiling: dirt and skin oil on a soft surface (our judgement) | Hardness 1.5 to 2; the sheet: colourless when not coloured by impurities | Edge-on rule: a tinted skin at the faces, clear through the thickness | Handle it less; methods are on the cleaning guide, not here |
| Yellow, tan, brown or black through the whole crystal, there from the start | Impurities in the crystal, as the sheet records | Sheet: yellow, tan, brown, reddish brown, gray, black if colored by impurities | Edge-on rule: colour runs through the thickness, often zoned | Nothing; that is the specimen |
| A frosted, chalky or white face after washing, rain or a damp room | Surface dissolution in water | 0.24 g per 100 ml at 25 °C: 2.4 g per litre | Frosting follows where the water went; dry faces stay glassy | No more water; see the solubility page |
| Thin sheets lifting or peeling from a broad face | Cleavage on {010} | Perfect on {010}; flexible, inelastic | Flakes are flat, pearly and parallel to the broad face | Keep the flakes with the specimen, labelled |
| Splinters lifting along a satin spar wand | Fracture along the fibres | Sheet: splintery parallel [001]; silky if fibrous | Splinters run the length of the fibres, not across | Support it along its length when stored or packed |
| An opaque, white, crumbly zone after heat | Loss of crystal water | ILO-WHO card: 100 to 150 °C; complete loss is 20.93% of the mass | Opaque through, not just frosted; a heat source in its history | Record it; no restoration is offered here |
| An even, bright colour on satin spar, deepest in cracks | Dye | The IGS lists dyeing, heat treatment and surface coating among gypsum's typical treatments | Colour concentrated in cracks and along fibres | The dyed-crystal checks |
| An oily or greasy look | The sheet's pearly lustre on {010}, or a residue on top | Sheet: pearly on {010} cleavages, silky if fibrous | A fresh cleavage flake shows the same sheen if it is lustre | Nothing, if it is lustre |
Why is my selenite turning yellow? What the gypsum sheet says about colour
Selenite has no sheet or IMA entry of its own: it is gypsum, and the Handbook of Mineralogy sheet for gypsum records colour as an impurity effect. The line is Colorless, white; if colored by impurities, yellow, tan, blue, pink, brown, reddish brown, gray, black; colorless in transmitted light. So yellow, brown and black selenite all exist naturally, and the question for a specimen that has changed is whether the colour is in the crystal or on it.
Method. The 176 sheets in this site's reference set were converted with pdftotext -layout and each Color field was extracted by script; all 176 print one. Five name impurities or inclusions as the source of colour: anhydrite, cerussite, gypsum, hemimorphite and quartz. The count is of what the sheets say, not of which minerals are coloured that way. Gypsum is one of 14 sheets with SO4 in the formula, and its 1.5 is the lowest hardness figure among the 12 of those that print a number.
The edge-on rule, stated once: look through the crystal edge-on against white paper. Colour that runs through the thickness, zoned or as inclusions, belongs to the crystal. Colour confined to a thin skin at the faces, or strongest where the piece is held, is on it; that pattern is our judgement of what surface soiling looks like on a soft, pearly surface, not a measured result. How to clean a specimen is on how to clean mineral specimens; this page gives no method, and the solubility page explains why water is not a neutral choice for gypsum.
Why is my selenite flaking? One perfect cleavage and a soft, bendy mineral
Selenite flakes because gypsum has perfect cleavage on {010}, the broad face of a typical crystal, and a tenacity the sheet records as flexible, inelastic. The sheet also gives a distinct cleavage on {100} and a third on {011}, with no quality stated; the indices here were read from the rendered sheet, not from extracted text. Pearly lustre on the {010} cleavages is what a peeled face shows. A flake bent past its limit stays bent rather than springing back.
Counted across this site's reference set: 163 of the 176 sheets print a numeric hardness, and 17 of those top out at 2 or below; four have a lower top than gypsum's 2: talc, molybdenite, pararealgar and arsenolite. Eighteen sheets record flexibility somewhere, matched ligature-aware because 4 of them print the fl as a stray glyph. Nine combine a perfect cleavage, a flexible tenacity and a hardness no higher than 2: covellite, graphite, gypsum, molybdenite, portlandite, pyrophyllite, stibnite, talc and vivianite.
Hardness 1.5 to 2 against calcite's 3. Calcite, which selenite is often shelved beside, is hardness 3 on its sheet, and the International Gem Society notes gypsum can be as low as 1.5 in some directions. Satin spar breaks differently: the sheet records fracture splintery parallel [001], so a wand splinters along its fibres. The general distinction between cleavage and fracture is on cleavage, parting or fracture, and hardness testing on how to test mineral hardness properly.
Why is my selenite chalky or white? Water and heat, with the numbers
Gypsum dissolves in water at 0.24 g per 100 ml at 25 °C, on the ILO-WHO International Chemical Safety Card for the mineral. That is 2.4 g per litre. In our judgement, uneven loss from a wetted face is what turns glassy selenite frosted where the water went while dry faces stay glassy. The worked example, a litre of water against a 15 cm wand, is on the solubility page and is not repeated here.
Heat is the other route to white. The same card gives an apparent melting point of 100 to 150 °C and notes that it is caused by loss of crystal water. Worked with standard atomic weights, CaSO4·2H2O is 172.16 and its water 20.93% of the mass, which reproduces the sheet's H2O column; losing 1.5 of the 2 water molecules, to the composition of bassanite, Ca(SO4)·0.5H2O on the IMA list, is 15.70%. The sheet's name line says gypsum is named from the Greek for calcined gypsum used as plaster: the same reaction. This page does not say what temperature a shelf, lamp or window reaches.
Damp rooms matter for a mineral that dissolves. Humidity bands for soluble and hydrated species are set out on how to store soluble mineral specimens and how to store a collection in a damp house; they are not repeated here and have not been tested on gypsum for this page.
Why is my selenite turning black or brown, and is it even selenite?
Black and brown are both on the gypsum sheet as impurity colours, so a dark crystal can be natural. A clear crystal that darkens on a shelf is, in our judgement, collecting dust and grease, and the edge-on rule above separates the two. Iron staining as a separate problem is on how to remove iron stain from a specimen.
Bright, even colour on satin spar deserves a second look. The International Gem Society lists dyeing, heat treatment and surface coating among gypsum's typical treatments, and notes occasional heating to improve colour or prepare for dyeing. Dye tends to gather in cracks and between fibres; the checks are on how to tell if a crystal is dyed. A piece that tests well above the sheet's 1.5 to 2 on a proper hardness test is worth checking against the fake-crystal page.
A white crust that grows is a different problem, set out on why is my specimen growing a white crust. The damage and conservation pages are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.