Really? Where I'm from, the first thing mages learn is generally a light spell. Making solid matter from magic... I don't think even the most powerful mages or sorcerers could do such a thing. [Create the semblance of solidity, as in some types of Sendings perhaps, but such a construct would only last until the spell faded- and it certainly wouldn't grant true sustenance. Experimentally, Sabriel puts one of the crystals in her mouth.]