She insisted on this part, and my father later backed her up: the magic was not just that the drum of a solid wood object without batteries rotated but that it did so only when unobserved, stopping whenever observed.

NEWYORKER: All That

That means that an object under the stack - in the team's experiments, a block of wood about 10cm long - would not "hear" the sound, and any attempts to locate the object using sound waves would not find it.

BBC: Acoustic 'cloaking device' shields objects from sound

Made from an offshoot of the Tree of the Knowledge of Good and Evil in the Garden of Eden, the wood takes various forms across the millennia, becoming an object of reverence, desire and fear for various crowned heads until the early seventh century, when a Christian emperor humbly carries it to its sanctuary in Jerusalem.

WSJ: A Heavenly View From On High | Basilica of Santa Croce | Agnolo Gaddi | By Francis X. Rocca