I don't know much about these, but I believe that is a George III half Guinea. Now I believe some were classified as tokens and made of brass or whatever metal was used to strike coins at the time and others were gold. I think the gold ones had a different shield on the reverse that yours.
This looks similar to yours found on eBay and it's not gold.
http://www.ebay.co.uk/itm/George-3r...hash=item4ad93c7d32:m:mSGKPwSWqiDw_yjhuJP1QGA
Did you dig this coin? It looks in great shape?