What causes colour in minerals and gemstones?
Mineral and gemstone colour is caused by a range of physical and chemical mechanisms — trace element impurities, crystal-structure defects, oxidation state and light-interference effects — meaning that chemically identical minerals can appear in very different colours.
Domain: MineralsDomain: GemstonesDomain: RocksMechanism: Mineral pigmentMechanism: OxidationMechanism: Weathering
Unlike a gemstone coloured by a mineral pigment, opal's flashes of colour come from microscopic silica spheres stacked inside the stone that diffract light — structural colour in a mineral, the same underlying physics as a butterfly wing.
An opal's flashes of colour shift completely with viewing angle and lighting; this photograph captures one specific angle, not the stone's only appearance."Opalescence of the opal" by Masha Milshina, CC BY 4.0, via Wikimedia CommonsLicence details
- Source:
- Wikimedia Commons
- Creator:
- Masha Milshina
- Licence:
- CC BY 4.0 — free to share and adapt commercially, provided the work is attributed.
- Reviewed:
- 2026-07-24
This boulder's colour is hematite, iron(III) oxide — the same mineral that gives red ochre earth pigments their colour, and the reason iron-rich soil so often looks red or rust-brown.
The original photograph includes roadside signage, snow and a photographer's shadow around the boulder; this palette was picked from the rock surface only, not the surrounding scene."Bengala Akanesawa red rock" by Aomorikuma, CC BY-SA 4.0, via Wikimedia CommonsLicence details
- Source:
- Wikimedia Commons
- Creator:
- あおもりくま (Aomorikuma)
- Licence:
- CC BY-SA 4.0 — free to share and adapt commercially, provided the work is attributed and any adaptation is shared under the same licence.
- Reviewed:
- 2026-07-24
How this colour is made
Mineral pigment
Trace metal ions or compounds within a mineral's crystal structure absorb specific wavelengths of light, giving the mineral its characteristic colour — the same chemistry behind natural earth pigments like ochre.
Oxidation
A chemical reaction with oxygen changes a material's surface composition and, with it, its colour — copper turning green (verdigris) and iron turning red-brown (rust) are both oxidation.
Weathering
Prolonged exposure to water, air and temperature change gradually alters a material's surface chemistry and texture, which changes how it reflects light — often making a polished surface look duller and paler over time.
Trace amounts of transition-metal impurities, such as chromium or iron, are responsible for the colour of many gemstones — for example, trace chromium is responsible for the green of emerald and the red of ruby, even though both are otherwise the same base mineral family in ruby's case (corundum) or a different one in emerald's (beryl).
Gemstones · confidence: high
Oxidation of iron-bearing minerals is responsible for many of the red, orange and brown colours seen in rocks and soil, including the reddish colouration common in iron-rich rock formations.
Rocks · confidence: high
Some gemstone colour, such as the play of colour in opal, is produced by light interference from microscopic internal structure rather than by a chemical pigment — a mineral analogue of the structural colour seen in some animals.
Gemstones · confidence: high
Weathering — the long-term chemical and physical breakdown of rock at the earth's surface — can substantially change a rock's original colour over time, for example turning grey iron-bearing rock progressively more orange-brown as it oxidises.
Rocks · confidence: high
Related colour profiles
Practical applications
Related questions
Frequency and rarity claims above are scoped to a specific domain, geography or taxonomic group — none are presented as a single universal ranking. See methodology.