Wow, Roger! If it is genuine, you have an amazing coin and a very valuable coin. Nice!
Gold pounds and half pounds were issued by Queen Elizabeth I way back in 1594 to 1596. The Queen herself reigned from 1558 to 1603. (Today's Queen is Elizabeth II, not Elizabeth I.)
These coins sell retail for prices between $5000 and $15000 US dollars. They are very special coins and each coin stands on its own merits. The value of your specimen will depend on many factors, including wear, damage, eye appeal, and source of authenticity. If you would like to send a picture, we might be able to give you a decent estimate. Use the Contact Us link to start an e-mail exchange.
NEVER CLEAN A COIN. CLEANING RUINS VALUE.
The magnificent coin in our picture comes from Ira and Larry Goldberg Auctions. It is in such beautiful condition, it would sell near the top of our $5000 to $15000 range.
Both pounds and half pounds reach these high values. Approximate weights are:
HALF POUND: 5.5 grams total coin weight
POUND: 11.0 grams total coin weight
I am a little concerned, Roger, that you say your coin is dated 1591. If a date like this appears on your coin, it may be a counterfeit. Genuine coins do not have dates. Use the Contact Us page if you think your dated coin may be genuine.