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: ‘the uncomely stone, the small or paltry stone’
Campbell identifies the precise textual source and alchemical meaning of lapis exilis as Wolfram’s name for the Grail, linking Arthurian legend directly to the philosophers’ stone tradition.
, Creative Mythology: The Masks of God, Volume IV, 1968thesis