There are some clues that seem to show that the Holy Grail is actually a scrying stone and not the cup related to The Last Supper with Jesus. Here is
my trail of thought:
First of all, mediums have used Scrying
stones for thousands of years in places like Ireland (Beryl),
Yucatan (crystals), and pre-Islamic Persia. Scrying stones today are in popular media shown as crystal balls, but many sort of materials have been
used, even dark water or ink, the only rule seem to be shining, luminous or reflective surface to let a medium, magician, or psychic to gain knowledge
Here is how the Encyclopedia Britannica describes scrying:
Divination of distant or future events based on visions seen in a ball of rock crystal. Divination based on an analysis of reflections in water, on
polished metal, or on precious stones was practiced by early humans, who probably interpreted these phenomena as a vision of the spirit world. Scrying
became widespread by the 5th century AD and was condemned by the medieval Christian church as the work of the devil.
So, Scrying has been in use since ancient times. Now over to the 10th century.
Cup of Jamshid was said to be filled with an elixir of
immortality and was used in scrying. As mentioned by Ali-Akbar Dehkhoda, it was believed that one could observe all the seven heavens of the universe
by looking into it. It was believed to have been discovered in Persepolis in ancient times. The whole world was said to be reflected in it, and
divinations within the Cup were said to reveal deep truths. Sometimes, especially in popular depictions such as The Heroic Legend of Arslan, the cup
has been visualized as a crystal ball. Helen Zimmern's English translation of the Shahnameh uses the term "crystal globe".
Here we can see that there seems to have been an evolution from some kind of crystal or stone used for Scrying to a cup with some dark liquid. In
archeology and culture we often see that items of importance seem to gain complexity and symbolism over time - this may have happened here too.
Now, lets move on to the 12th century Europe, here the legend of The Holy Grail
first appear, and
as many have suggested before me it seems that the Cup of Jamshid is the origins for the real Holy Grail myth and not the cup associated with the Last
Supper with Jesus Christ. The Holy Grail also seem to have many of the same traits, but here the main purpose seem to be gaining immortality - whereas
the Cup of Jamshid merely was filled with elixir of immortality the purpose of scrying. Also, it is worth noting that according to legend King Arthur
was scrying himself.
It seems that it is plausible that the Holy Grail, Cup of Jamshid, and Scrying stones are all connected over the millennia’s - a mythical tool used
by mediums and psychics that evolved into the 12th century Holy Grail myth.