I'm reading a very good book on the history of Greece, A History of Greece To 322 BC by NGL Hammond. Its a great work and very comprehensive. What's more its not filled with all the asinine cultural and political references and ax grinding that modern day historians seem bent on including in their studies (one historian i recently read wandered off his discussion of Sparta and its society to discuss fox hunting in modern day England).
At any rate, just finished up Hammond's bit on coins.
Accordingly the first coins were pieces of iron called "dumps" that were marked as to type...solid or plate and later marked to show who had created them. These were used in trade and barter. It was a king in Lydia who first began to stamp his precious metals with his sign, a lion. The metal of choice? Electrum.
These began circulating as trade goods and were joined by other states in the Aegean (Lydia was in modern day Turkey). These eventually became a medium of exchange, especially when the Persians got involved and began minting beautiful and carefully weighted gold coins.
And there you have it. The first coins.
Here is the Lydian coin.