hens' teeth

Something very scarce (because hens have no teeth). Typically used in phrases like "rarer than hens' teeth." I can hardly believe your brother got an A on his final exam—grades like that are rarer than hens' teeth for him! Please, who would I even ask to the prom? A high school boy who isn't a doofus is as rare as hens' teeth. Support for the president is rarer than hens' teeth in this part of the country.