George III was King of England from 1760 to 1820 (he was King during the American Revolution), so the 1700 date on Darrell's piece is a stretch. This is not a coin, but a token. It does not appear in the coin catalogs, but one can find these online if you are persistent. Here is what Sven Andersson says (translated from Swedish) on Unga Fakta:
'It is a game of money, a copy in brass of George III (King of England) gold coins. It was used as a chip in various card games and board games. It produced quite a few of them and spread to various countries.'
So Darrell's piece is not an actual gold guinea from Britain, but a gaming token with similar appearance. You can see from our secondary picture of a *real* gold guinea that the cryptic, single-letter inscriptions are different. See this page if you have a genuine coin.
CoinQuest thanks LaMoneta.it for use of their image of the token.
Now, with identification tidied up, we can move to value. Ugh. The coin catalogs indicate nice, fat values for a genuine gold George III guinea (and half guinea, which looks the same). These values start around $100 US dollars and rise toward $1000 for fully uncirculated coins.
But the Spade Guinea token is another story. Go on to eBay and do a search on 'George III Token.' You will see a myriad of tokens, many called Spade Guineas, with many variations on the same theme. Dates start as early as 1700 and continue all the way to today! Selling price varies with age and condition, but hovers around $5 to $25 US dollars retail price. A dealer would pay less than one-half retail to buy this token from you.