In Indiana Jones and the Last Crusade, the true Holy Grail is made from wood.
The film’s central test requires Indiana Jones to choose one cup from a chamber filled with elaborate vessels. Walter Donovan selects a richly decorated golden cup, assuming that the Grail would belong to a king. The choice is fatal because the cup is not the humble vessel used by Jesus.
Indiana instead identifies a plain wooden cup as the most plausible object for a travelling carpenter. The Grail Knight confirms that this is the correct choice, while Donovan rapidly ages after drinking from the wrong cup.
This detail belongs to the film’s fictional mythology, not to a proven historical relic. The Valencia Chalice and other objects have been proposed as real-world Grail candidates, but none has been conclusively authenticated as the cup from the Last Supper.