A pollen corona crowns the castle of Giulietta at Montecchio Maggiore, Italy. An image by Francesco Della Stella Dalla Stella.
Pollen grains from a particular tree are all the same size and shape. Thatís perfect for making a corona. Each grain diffracts the sunís rays in the same way to give us a sharp diffraction pattern in the sky with undiluted colours.
Ordinary water droplets in clouds are less good. They have a range of sizes. Even worse, their sizes can change dramatically across the coronaís angular extent. That gives us broken coronae and iridescence. Still spectacular but in a different way.