King George III minted these coins in gold from 1801 to 1813. The two denominations covered here: half guinea and one guinea, have the same portrait of the King and identical inscriptions, but the reverse side is different -- the guinea and half guinea have larger coats-of-arms and include the 'HONI SOIT QUI MAL Y PENSE' inscription on a circular ribbon. That inscription, by the way, warns against the ills of sour thinking.
Also, the guinea weighs in at 8.35 grams, the half guinea at 4.18 grams. The smaller one-third guinea and one-quarter guinea weigh in at 2.78 and 20.9 grams. Gold content is:
GUINEA: 0.246 troy ounces gold
HALF GUINEA: 0.123 ounces
ONE-THIRD GUINEA: 0.082 ounces
ONE-QUARTER GUINEA: 0.062 ounces
Our page on 1/3 guineas appears at this link [PRESS HERE].
CoinQuest thanks respected Australian coin dealer Noble Numismatics for use of their photos on this page. Nice coins!
These coins are somewhat rare and valuable. Counterfeits exist of these coins, so be wary. If you believe you have a genuine coin, it would be a good idea to take the coin to a coin shop, where a professional can look at the coin in-hand and (probably) determine its authenticity.
This is what the catalogs say about approximate values for these coins:
worn: $1200 US Dollars approximate catalog value
average circulated: $2800
well preserved: $5200
fully uncirculated: $8500
worn: $300 US Dollars approximate catalog value
average circulated: $450
well preserved: $800
fully uncirculated: $1300
Coins dated 1811 are more valuable: multiply the above values by 2.
Coins dated 1813 are slightly more valuable: multiply the above values by 1.5.
The above values are only very approximate catalog values. With older gold coins like this, you have to look at each separately. You can use the above tables as very general guidelines. Be cautious when buying coins like this online - if the deal seems too good to be true, it probably is!
If you have one of these coins completely worn down to only be just barely identifiable, and it is not one of the less common dates, the coin will only be worth the value of the gold it contains. Using a site like kitco.com, we can calculate the gold value of these coins. Gold currently sells at around $1400 per ounce. Multiplying by the gold weight of the coin, we can find the value. Tomorrow gold will sell at a different value, so be sure to look it up.
At today's prices, a half guinea has $1400 x 0.123 = about $170 dollars of gold in it.