Colour and Pleochroism: Why a Stone Changes Shade with the Angle
A mineral’s colour comes from its composition, from impurities, from defects or from optical effects. Pleochroism is the change of colour depending on the direction of observation.
Key points
| Intrinsic colour (idiochromatic) | Due to a constituent element: malachite, peridot, rhodonite |
|---|---|
| Impurity colour (allochromatic) | Ruby (chromium), amethyst (iron and natural irradiation) |
| Optical colour (pseudochromatic) | Opal, labradorite: interference, diffraction |
| Pleochroism | Colour that varies with direction: dichroism (2) or trichroism (3) |
| Stones without pleochroism | Cubic and amorphous: garnet, spinel, diamond, glass, opal |
| Striking example | Iolite: violet-blue, light blue, pale yellow |
Where does a mineral’s colour come from?
A mineral looks coloured because it absorbs certain wavelengths of white light and lets the others pass through or reflects them. What it absorbs depends on its atoms, its impurities and the defects in its lattice. Mineralogists distinguish three main causes.
| Family | Cause | Examples |
|---|---|---|
| Idiochromatic (“coloured by itself”) | An essential element of the formula gives the mineral its colour | Malachite and azurite (copper), peridot (iron), rhodochrosite and rhodonite (manganese), sulphur |
| Allochromatic (“coloured by something else”) | Impurities present in very small amounts in a crystal that is colourless when pure | Ruby (chromium in corundum), emerald (chromium or vanadium in beryl), aquamarine (iron), citrine (iron) |
| Pseudochromatic | Physical effects of light, without any pigment | Opal (diffraction), labradorite, moonstone, iridescence |
To these are added colour centres: defects in the lattice created by natural irradiation. Amethyst (iron and irradiation) and smoky quartz (aluminium and irradiation) are examples. And inclusions can colour a stone on their own: green aventurine owes its tint to flakes of green mica (fuchsite). For rose quartz, the origin of the colour is still debated: traces of titanium, manganese or iron, or microscopic fibres, depending on the studies.
Colour can be modified
Several common treatments change colour and must be disclosed:
- heating: amethyst turns yellow and becomes citrine, greenish aquamarine becomes blue, brownish tanzanite turns blue to violet;
- irradiation: a colourless topaz turns blue; professionals check residual radioactivity before sale;
- dyeing: agate, howlite, jade or chalcedony can be coloured throughout;
- coating: “aura” quartz receives a metallic layer.
A sign that a stone has been dyed: vivid colours that do not exist in nature and that collect in the fissures. See how to recognise dyed agate and our commitments.
Pleochroism: one colour, several shades
In a crystal that is neither cubic nor amorphous, light does not travel in the same way in every direction. Absorption also varies, so different shades are seen depending on the direction in which one looks. This colour change is pleochroism.
- Uniaxial crystals (tetragonal, hexagonal, trigonal) show at most two colours: this is called dichroism.
- Biaxial crystals (orthorhombic, monoclinic, triclinic) can show three: this is trichroism.
- Cubic crystals and amorphous materials (glass, opal) show none, in principle.
See also the trigonal, orthorhombic and cubic crystal systems.
Examples of pleochroic stones
| Stone | System | Colours depending on direction | Intensity |
|---|---|---|---|
| Iolite (cordierite) | Orthorhombic | Violet-blue, light blue, greyish pale yellow | Strong |
| Andalusite | Orthorhombic | Olive green, brownish red, yellow-green | Very strong |
| Tanzanite (zoisite) | Orthorhombic | Blue, violet-purple, yellow-green | Strong |
| Alexandrite | Orthorhombic | Green, red-orange, yellow | Strong |
| Kunzite | Monoclinic | Almost colourless, pink, violet | Strong |
| Ruby | Trigonal | Purplish red, orangey red | Medium |
| Blue sapphire | Trigonal | Intense blue, blue-green | Medium to strong |
| Green or brown tourmaline | Trigonal | Dark green, light yellow-green | Strong |
| Emerald | Hexagonal | Blue-green, yellow-green | Medium |
| Garnet, spinel, diamond | Cubic | A single colour | None |
This last row is useful: a red spinel, being cubic, shows no pleochroism, whereas a ruby, being trigonal, does. A red “stone” that never changes can therefore be a spinel, a garnet or glass. This is a clue, never a proof: internal stresses can produce a faint effect in cubic crystals. See our pages on iolite, tanzanite, kunzite and spinel.
Do not confuse it with other effects
- Change of colour: alexandrite looks green in daylight and red under an incandescent lamp. This is an effect of the light source, not of the viewing angle. Alexandrite and pleochroism can coexist in the same stone.
- Dispersion (“fire”): the splitting of white light into colours by a cut stone (very clear in diamond).
- Iridescence and play of colour: see lustre.
Observing pleochroism without equipment
Gemmologists use a dichroscope, a small tube containing a piece of calcite that shows two images side by side. You can improvise:
- Place a transparent or translucent stone on a white background that is evenly lit (a sheet of paper in daylight).
- Rotate it slowly and tilt it: check whether the shade changes clearly depending on the direction.
- Better still: place it on the white screen of a computer or phone (whose light is polarised) and turn it; also look through polarising sunglasses. On iolite, the colour shifts from blue to pale yellow.
Pleochroism has also been known for a long time: a medieval Icelandic account mentions a “sunstone” used by navigators. It has been suggested that this was cordierite (iolite) or calcite, which polarise the light of the sky. This hypothesis remains debated and is not supported by direct evidence of such use.
What to keep in mind when choosing a stone
Strong pleochroism is an asset for the cutter: it guides the orientation of the stone to obtain the most beautiful colour (as with tanzanite, for example). For the buyer, it explains why the same iolite can look blue face-on and grey from the side: this is not a defect. The photos on our pages are indicative; see our commitments. For the reading of colours in symbolic tradition, see instead our guides by colour: the science of colour and its symbolism are two distinct subjects.
Frequently asked questions
What is pleochroism?
It is the change of colour of a crystal depending on the direction in which it is viewed. It comes from the different absorption of light in each direction of the crystal.
What is the difference between dichroism and trichroism?
Dichroism: two colours, in uniaxial crystals. Trichroism: three colours, in biaxial crystals.
Is glass pleochroic?
No, in principle. Glass is amorphous, so its colour is the same in every direction. This is a clue for telling glass apart from ruby or tanzanite.
Why is amethyst purple?
Because of iron and lattice defects created by natural irradiation. When heated, it can turn yellow and become citrine.
Is a stone that changes colour with the light pleochroic?
Not necessarily. Alexandrite changes with the light source, which is a different phenomenon from pleochroism.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Mineralogy for beginners.