in Thoreau’sJournal:

Once or twice of late I have seen the mother-of-pearl tints and rainbow flecks in the western sky. The usual time is when the air is clear and pretty cool, about an hour before sunset.

Yesterday I saw a very permanent specimen, like a long knife handle of mother-of-pearl, very pale with and interior blue, and rosaceous tinges. I think the summer sky never exhibits this so finely.