Surprisingly, the earliest stories don't describe a cup at all. In Chrétien de Troyes’ unfinished romance Perceval, the Story of the Grail (circa 1190), the Grail is not a chalice but a —a shallow, jewel-encrusted platter bearing a single Mass wafer that sustains the wounded Fisher King.
The Holy Grail endures because it is infinitely interpretable: The.holy Grail