On the reverse, just to the left of the eagle, are 4 Greek characters. Those characters provide the date; which in turn can provide which Ptolemaic King your coin was issued under. There appears to be a L II; but without having the bottom character on flan; I'm not sure if there's any way of knowing. It looks authentic to me and probably a tetradrachm; though similar looking didrachms were issued. Do you have a weight?
These coins are Greek. They were minted from around 350-30BC if I am correct. That is really all I know about this particular piece. It is silver.
The edges of your coin appear to be cut after strike, which would explain the weight and verify that it is fake.
If the coin weighs is 10.1 grams, then that concerns me. A tetradrachm should weight around 14g, a didrachm around 7g. It is possible that your coin may be silver plated rather than solid silver. That would mean that it's a counterfeit. It might be an ancient counterfeit that circulated around the same time as the real coins, which would still make it interesting and collectable, or a modern counterfeit, which would make it more or less worthless.
