The core of a wand greatly influences how easy it is to master and how easily it will bend its allegiance to a new owner. Is it ever mentioned what core was used in the elder wand? I'm pretty sure it's never mentioned in the books, and in “Tales of Beedle the Bard” it is just said that the wand was fabricated and there's no other detail. Is it mentioned in other official sources, such as author interviews?
Alternatively, if this does not have a definite answer, I would also be interested in a logical speculation of what the core might be, based on what we do know about the various wand core types, their characteristics, and the characteristics shown by the elder wand.