Yet another possible reading of the Holy Grail is as a Christianization of the Cauldron of Dagda (which, yes kids, is the same as the Black Cauldron), an object of Celtic legend which was supposed to give forth food eternally. The fits nicely with the reading of the Grail Myth which sees it as a story of the restoration of fertility and nature's fecundity: The Fisher King has been "pierced through the side", which is almost certainly a medieval euphemism for his having been castrated. Becuase of this, the Grail Kingdom has become a wasteland, and only a new young king, Perceval, and the intervention of the Cup of Plenty, can restore balance.

Of course, this is not the only interepretation. The idea of the Grail as the philosopher's stone also has many interesting implications. One common Latin name for the Grail as Stone is the lapis exillis, or "exiled stone" (at least roughly; the exillis part is bad Latin). In this version of the story, the Grail/Stone has been expelled from Heaven, which leads to the common modern reading of the Grail as meteorite. To a medieval reader, the matter would have been even more serious, though, because the Heaven which the Stone was expelled from was a literal one, making the Grail a piece of heavenly matter.

To an alchemist, especially one of a Gnostic bent, this made the Grail the most precious thing in the world. In this scheme, the idea of the Philosopher's Stone converting base metals to gold is a metaphor: Gold was the incorruptible, incorrodable metal, and as base metals could be transmuted to gold, so could flawed creation made whole again. With the the Grail in the right hands, Sin of Adam, or the failure of the Demiurge, could be rectified, man's mortal and flawed nature corrected, and the Kingdom of Heaven realized on Earth.

One can try to piece these readings into conspiracy theories of Merovingians and Knights Templar, but only at the risk of losing your mind completely. Some rabbit holes just go too far.