Figuration. Statue. Flint pebble rolled by the Storebaelt, Nyborg, Fyn.

Titan Kronos, fearful of the prophecy that his children would overthrow him, ate them whole upon their birth. Until Rhea, his partner, gave Kronos a stone that he mistook for the infant god Zeus.

Having returned from Mount Ida where he grew to maturity, Zeus toppled Kronos, released his brothers and sisters, and set the stone at the sanctuary at Delphi.

Here is Hesiod, Theogony 498–500:

τὸν μὲν Ζεὺς στήριξε κατὰ χθονὸς εὐρυοδείης
Πυθοῖ ἐν ἠγαθέῃ γυάλοις ὕπο Παρνησοῖο,
σῆμ᾽ ἔμεν ἐξοπίσω, θαῦμα θνητοῖσι βροτοῖσιν.

“Zeus fixed it in the broad-wayed earth, at holy Pytho beneath the hollows of Parnassus, to be a sign thereafter, a wonder to mortal human beings.”

Here is what Herakleitos (DK B93) said of the place, its god (Apollo) and the oracle at Delphi:

… οὔτε λέγει οὔτε κρύπτει ἀλλὰ σημαίνει

“… neither speaks nor conceals, but gives a sign”