Gemstone of Paradise:  The Holy Grail In Wolfram's Parzival by G. Ronald Murphy

Did you know that the Holy Grail was a rock?

This is an analysis of a medieval account of the Holy Grail, wherein it is indeed a rock -- and as the author deduces, an altar stone, which ties it back to the main thread of Holy Grail stories.  It reworks Chretain's tale with few but significant tale, and unlike Chretain's, was finished.

The author goes into various themes that interplay.  The stuff on gemstones is particularly neat.  Do you know that absolutely perfect rubies and garnets glow in the dark like live coals?  That emeralds help eyesight, and the very best ones are found in gryphons' nests?  That gemstones come from rivers?  That, actually, is where a lot of them come from to this day, though at the time they didn't know it was because they had been washed out of their rocky matrix.  It also goes into such things as the Crusades and the Holy Sepulcher, and the significance of the characters in the story.