|Pollen grains floating in a clear blue sky have scattered and diffracted sunlight to form the delicate ringed corona. Rosy pinks and pastel straws border the inner white disk of the aureole. The first ring starts with cyans altering through greenish hues, rusts and hints of purple to the outer edge.
Most coronae are produced by cloud water droplets. Ice crystals also make them as can pollen grains, algae on pond surfaces or any reasonably monosized particle cloud. All but water droplets are semi or completely opaque but this does not matter because coronae result largely from light scattered by the particle surface.
Imagine, as in the diagram at right, plane light waves scattered from two points on the particle skin. Each point produces a set of outgoing spherical waves. Where the waves overlap they interfere. Where both wave crests coincide there is light. When the waves are out of phase there is darkness. This is the "near field". At a long distance away, the "far field", the result is the corona diffraction pattern.
Pollen corona can be quite non circular with bright regions. This early corona is not so because the hazel grains are roughly spherical. Some other pollen types have prominent air sacs to enable them to drift long distances in the wind. The sacs orient the grains and give rise to asymmetric light scattering.