the Holy Grail, even in Chrétien’s text, was neither a bowl nor a cup, not the chalice of the Last Supper… but, as Professor Loomis reminds us, ‘a dish of considerable size.’ … In Wolfram’s text the Grail is a stone. ‘Its name,’ he declares, ‘is lapis exilis,’ which is one of the terms applied in alchemy to the philosophers’ stone
Campbell establishes the Grail’s radically unstable material identity, arguing that Wolfram’s alchemical lapis exilis reading signals a deliberately non-ecclesiastical, esoteric symbolic register.
, Creative Mythology: The Masks of God, Volume IV, 1968thesis