Not everyone believes that the Grail is a mere piece of crockery. In some stories, the Holy Grail is described as a stone -- either a stone tablet containing the wisdom of the ages or a rock with mysterious powers. Some writers have even claimed that the Grail is a meteor.
And anytime you start talking about stones with mysterious powers, dabblers in esotericism will start mumbling about the holy grail of alchemy -- the Philosopher's Stone, which had the power to either (A) turn lead to gold or (B) grant eternal life.
Even more interesting is the theory that our concept of the Grail is the result of a mistranslation -- in French, "Holy Grail" is "san greal," but if you shift one letter over, you get "sang real," or "Royal Blood." According to this theory, recorded in detail in a book called "Holy Blood, Holy Grail" by Michael Baigent, Henry Lincoln, and Richard Leigh, the Holy Grail is actually a metaphor for the bloodline of Jesus Christ -- the direct descendants of Jesus and Mary Magdalene who, we are told, escaped to France to become the semi-legendary Merovingian kings, the Priory of Sion, the Rosicrucians, the Knights Templar, the Freemasons, and, of course, the Illuminati.