A bit late in the day, but I couldn't not share this coin, which comes at an interesting point for Athenian coinage, as ca. 525 BC as Reverse image was included for the first time, which allowed for a dedicated Obverse image: the Gorgon.
Indeed, the tetradrachm itself as a denomination was introduced at this time. Weighing around 17.28g, this was a high denomination coin, one that seems to have been designed for high-value transactions.
The Gorgon's head is particularly striking, though somewhat marred on this example by a secondary cut, doubtless a means of testing the precious metal content of the coin. 
The Gorgon's head was used as a symbol of Athens herself, associated with the aegis that Athena wore, and suggests the advertising of an Athenian monetary standard beyond the bounds of Attica.
Image: Athena Giustiniani: Roman copy of a Greek original ca. 400 BC (Vatican Museums)
Image: Athena Giustiniani: Roman copy of a Greek original ca. 400 BC (Vatican Museums)
It seems likely, from metallurgical analysis, that these changes coincided with the exploitation of the silver mines at Laurion, thus serving to commodify Athenian silver to a great extent.
The design, however, was relatively short-lived, with the Gorgon replaced ca. 510 BC with a standardised head of Athena, turning an oblique reference into a direct one, with her owl as an added identifier.
Image: ANS 1957.172.1030
Image: ANS 1957.172.1030
