It appears to be a silver coin from the Thraco-Macedonian tribe of the Orreskioi, dating to the early 5th century BC (the time of the Persian Wars). It looks very similar to the one here, and you can read the first letters ΟΡΡΗΣ Orrês- on your coin (the capital sigma Σ is written backwards). The Orreskioi were from a region with its own silver and gold mines, so they adopted coinage relatively early compared to many of their neighbours. Judging by the thickness, the coin is probably a Phoenician octodrachm (the word "Phoenician" here only indicates the weight standard used to measure the amount of silver used to strike one 8-drachma piece). You can verify this with sufficiently sensitive kitchen scales; it should weigh about 28g.