Short answer
A photonic crystal is a material whose refractive index repeats in two or three dimensions at the scale of light's wavelength, so that some wavelengths cannot travel through it in some directions and are reflected instead. Peacock barbules hold a two-dimensional lattice of melanin rods, several butterflies grow a three-dimensional gyroid of chitin, a weevil has a diamond-type lattice, and chameleons carry a lattice of guanine nanocrystals whose spacing they can change.
A multilayer repeats in one direction, through its thickness. Repeat the pattern in a second direction and it becomes a two-dimensional photonic crystal; repeat it in all three and light meets a periodic structure whichever way it travels. Precious opal is the familiar mineral case: it consists of highly ordered silica spheres, typically 150–400 nm across, and its play of colour follows a Bragg-type relation between the reflected wavelength and the sphere diameter. Opal is offered here as an analogy only. Biological lattices are made of different materials by an entirely different process, and they resemble opal in their optics, not their origin.
In a peacock's train feather the colour sits in the barbules. Just beneath the barbule surface, rods of melanin are bound in keratin in a regular lattice with air channels between them. Simulations of that structure show a partial band gap in the direction normal to the surface: light in a band of frequencies cannot propagate inwards and is reflected. Two parameters do the work. Changing the lattice constant — the rod-to-rod spacing — shifts the reflected band and produces the blue, green and yellow regions. Reducing the number of lattice periods broadens it and adds further wavelengths, which gives the brown areas their mixed colour. One building plan with two adjustable numbers accounts for the whole eyespot.
The green scales of the green hairstreak and several other butterflies contain a network of chitin with the geometry of a single gyroid: a continuous, triply periodic labyrinth that mathematicians described before biologists found it. X-ray scattering on single scales of five species from two families confirmed the structure. The route to it is as striking as the result. Inside the developing scale cell, the cell membrane and the smooth endoplasmic reticulum fold together into a double gyroid, a form that lipid membranes and block copolymers adopt spontaneously. Chitin is then deposited into one of the two interleaved spaces, the cell dies and the rest degenerates, leaving a single gyroid of chitin in air. The butterfly does not carve the lattice; it lets membrane physics template it.
Photonic engineers also aim for single-gyroid lattices, and find them difficult to make at optical length scales.
A single perfect crystal is strongly direction-dependent, like one patch of opal. Yet the hairstreak's green and the weevil Lamprocyphus augustus's green barely change as the animal turns. The weevil's scales contain a diamond-based lattice of chitin and air, assembled as micrometre-sized single-crystal domains with different lattice planes facing outwards. Each domain reflects a slightly different colour in a slightly different direction; at the scale of an eye they blend into a steady green. The gyroid scales work the same way, with many small domains per scale. Order at the nanometre scale and disorder at the micrometre scale together give a saturated colour that is nearly independent of angle — a combination no single multilayer can provide.
Panther chameleons were long assumed to change colour by moving pigment. Work published in 2015 showed that the upper layer of their skin's iridophores contains guanine nanocrystals arranged in a lattice, and that the animal changes the spacing of that lattice: closer packing in relaxed skin reflects blue, which the overlying yellow pigment turns green, and wider spacing in excited skin moves the reflection towards yellow, orange and red. A second, deeper layer of iridophores holds larger, less ordered crystals that reflect broadly and strongly in the near infrared. The authors suggest that layer may offer passive thermal protection; that suggestion has not been tested in the cited work.
| Organism | Structure | Mechanism | Made from | Change with angle |
|---|---|---|---|---|
| Green hairstreak (Callophrys rubi) | Ventral wing scales | Photonic crystal (2D or 3D) | A single-network gyroid of chitin and air: a three-dimensional photonic crystal grown on a folded membrane template. | Weak. Each scale holds many small crystal domains in different orientations, which averages out the direction-dependence of any one domain. |
| Diamond-structure weevil (Lamprocyphus augustus) | Body scales | Photonic crystal (2D or 3D) | A diamond-based three-dimensional lattice of chitin and air, assembled as micrometre-sized single-crystal domains in different orientations. | Weak. Individual domains are direction-dependent; the mosaic of orientations makes the whole scale nearly angle-independent. |
| Indian peafowl, male (Pavo cristatus) | Train feather barbules | Photonic crystal (2D or 3D) | A two-dimensional lattice of melanin rods and air channels in keratin, in the barbule cortex; lattice spacing and number of periods set the colour. | Strong. Iridescent; hues shift as the train moves. |
| Panther chameleon (Furcifer pardalis) | Two layers of dermal iridophores | Photonic crystal (2D or 3D) | A lattice of guanine nanocrystals in the upper iridophores whose spacing the animal changes; a deeper layer of larger, less ordered crystals reflects broadly, strongly in the near infrared. | Weak. The skin's colour is governed mainly by lattice spacing; yellow pigment above converts structural blue to green. |
| Precious opal | Regular arrays of silica spheres | Photonic crystal (2D or 3D) | Highly ordered silica spheres, typically 150–400 nm across, forming a natural photonic crystal. | Strong. Shows a play of colour; the Bragg relation that describes it includes the angle of the incident light. |
| Organism | Colour | Proposed function | Evidence grade | What the evidence is |
|---|---|---|---|---|
| Green hairstreak (Callophrys rubi) | Matt leaf green | Concealment on foliage has been suggested for green gyroid scales. | Proposed, untested in cited work | The cited papers establish structure and optics; neither tests predation. |
| Diamond-structure weevil (Lamprocyphus augustus) | Green that barely changes with angle | None assigned in the cited work. | Not established | A structural and optical study. |
| Indian peafowl, male (Pavo cristatus) | Blue, green, bronze and brown eyespots | Female mate choice. | Comparative or correlational; contested | A 1991 study concluded that peahens prefer elaborate trains; a seven-year study of a feral population in Japan found no preference for more eyespots, symmetry or length; later work on 34 males relates success to eyespot colour. |
| Panther chameleon (Furcifer pardalis) | Green and blue at rest; yellow, orange and red when excited | Social signalling in contests and courtship; possibly passive thermal protection from the deep layer. | Comparative or correlational | A comparative study of dwarf chameleons associates capacity for colour change with signal conspicuousness and finds no support for background matching. Thermal protection is offered as a possibility. |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
The cortex of peacock feather barbules contains a two-dimensional photonic-crystal structure, and varying its lattice constant and number of periods accounts for the different colours of the train.
FactStrong evidence
Small-angle X-ray scattering on single wing scales of five butterfly species identified their chitin-and-air nanostructures as single-network gyroid photonic crystals, which develop from a double-gyroid membrane precursor within the scale cell.
Source: Structure, function, and self-assembly of single network gyroid (I4132) photonic crystals in butterfly wing scales; Gyroid cuticular structures in butterfly wing scales: biological photonic crystals
FactStrong evidence
The scales of the weevil Lamprocyphus augustus contain a diamond-based three-dimensional photonic crystal assembled as differently oriented single-crystal domains, which gives the animal a nearly angle-independent green.
Source: Discovery of a diamond-based photonic crystal structure in beetle scales
FactStrong evidence
Panther chameleons change colour by actively tuning a lattice of guanine nanocrystals in a superficial layer of dermal iridophores, while a deeper iridophore layer with larger crystals reflects a substantial proportion of sunlight, especially in the near infrared.
Caveat: Thermal protection by the deep layer is described by the authors as potential, not demonstrated.
Source: Photonic crystals cause active colour change in chameleons
FactStrong evidence
Natural precious opal is a photonic crystal of highly ordered silica spheres, typically 150–400 nm in diameter, and its play of colour follows a modified Bragg's law relating the reflected wavelength to the sphere diameter.
Caveat: A mineral analogy for the optics of biological lattices; no biological relationship is implied.
Source: Facile control of silica nanoparticles using a novel solvent varying method for the fabrication of artificial opal photonic crystals; Colour of precious opal
Reviewed 6 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.