As rare as hen's teeth

Be as scarce as hen's teeth (American & Australian slang):

to be very difficult or impossible to find.

http://idioms.thefreedictionary.com/as+rare+as+hen's+teeth

http://www.peevish.co.uk/slang/a.htm

As rare as hen's teeth? Not any more, say scientists.

If you thought hen's teeth were the rarest thing in nature, think again: researchers from Britain and the US have succeeded in growing teeth in a chicken.

Far from being rarer than students who turn up at 9am lectures or lecturers who like giving them, a hen with teeth does occur naturally, scientists based at the universities of Manchester and Wisconsin have found.

Dr Matthew Harris, the lead researcher from Wisconsin, told how scientists were able to isolate the usually latent genetic "pathway" that leads to tooth growth - and create the "initiations of early teeth".

http://education.guardian.co.uk/higher/research/story/0,,1716328,00.html

They said this had involved activating genes that had lain dormant for about 80 million years.

This could help research into replacing teeth lost by humans due to accident or disease, Prof Ferguson added.

http://news.bbc.co.uk/1/hi/england/manchester/4740726.stm

http://www.eurekalert.org/pub_releases/2006-02/uom-htn022206.php