They live from a Stone whose essence is most pure. If you have never heard of it I shall name it for you here. It is called Lapsit exillis. By virtue of the Stone the Pheonix is burned to ashes, in which he is reborn.
To von Eschenbach, the Grail was never really a material cup, but a jewel like the jewel in the lotus, a symbol of enlightenment, of something intangible and always beyond reach.
I must confess that the reason I know this is because I'm a bit of an opera fan. The work of von Eschenbach was a source for Wagner. Just the other day I was thinking how nice it would be to hear das Rheingold again, and the beautiful arias of the Faustian Alberich as he curses the gods.