1560s, from Middle French agathe (16c.), from Latin achates, from Greek akhates, the name of a river in Sicily where the stones were found (Pliny). But the river could as easily be named for the stone.
The earlier English form of the word, achate (early 13c.), was directly from Latin. Figurative sense of "a diminutive person" (c. 1600) is from the now-obsolete meaning "small figures cut in agates for seals," preserved in typographer's agate (1838), the U.S. name of the 5.5-point font called in Great Britain ruby. Meaning "toy marble made of glass resembling agate" is from 1843 (colloquially called an aggie).