What was engraved on the stones of Aaron's breastpiece?

Answer: The names of Israel's sons

Twelve stones in four rows were each engraved like a seal with the name of one of the sons of Israel, the twelve tribes.

“The twelve stones are to correspond to the names of the sons of Israel, each engraved like a seal with the name of one of the twelve tribes.”— Exodus 28:21 (BSB)