A material whose internal structure repeats at about the scale of light's wavelength, so that it reflects some colours strongly without containing any pigment of that colour.
The refractive index varies periodically in two or three dimensions, so certain wavelengths cannot travel through the lattice in certain directions and are reflected instead. Peacock barbules hold a two-dimensional lattice of melanin rods; several butterflies grow a three-dimensional gyroid of chitin and air; one weevil has a diamond-type lattice.
A photonic crystal is not always iridescent. A lattice built from many small domains pointing in different directions averages out the change with angle, which is how one weevil's scales stay nearly the same green from every side.
The play of colour in precious opal comes from highly ordered arrays of silica spheres, a mineral that works on the same optics as these biological lattices.
It is the most ordered kind of structural colour, and it explains why some of the most saturated blues and greens on animals cannot be extracted as a dye and vanish if the structure is crushed or filled.
A photonic crystal is identified by its geometry and confirmed by its optics. Small-angle X-ray scattering on single butterfly wing scales identified their chitin-and-air networks as single-network gyroids. For the peacock, a model of a two-dimensional lattice of melanin rods reproduces the colours of the train when two quantities are varied: the lattice constant, which is the repeat distance, and the number of periods. Reflectance measured at several angles then shows how far the colour moves with viewing direction.
Someone tries to extract the blue or green from an iridescent wing or feather.
There is no pigment of that colour to extract. The colour comes from the spacing of the lattice, so grinding the scale, or wetting it with a liquid that fills the air spaces, removes or shifts it.
A chameleon's colour change is explained as pigment being moved around the skin.
Panther chameleons shift colour by actively changing the spacing of a lattice of guanine nanocrystals in their upper iridophores. The pigment cells above filter the result but do not produce the shift.