(From Festus, s.v. lapidem silicem)
Oaths were often sworn to the Gods, employing them as the arbiters and punishers of transgressors of those oaths. They are sworn outdoors, so they me properly witnessed by the heavens. Those who are about to swear by Iuppiter hold a stone of flint, saying these words:
"Si sciens fallo, tum me Dispiter salva urbe arceque bonis eiciat ut ego hunc lapidem."
(Which, translated, means "If I knowingly deceive, then may Ivppiter throw me out of my property, keeping the city and the citadel safe, as I cast this stone.")
He then hurls the flint as far away from himself as possible.