All California gold coins need to be seen for an accurate assessment. The U.S. Mint never issued a octagonal coin of any denomination. Find a coin dealer in your area to take it to.
insurgent propaganda may portray their enemies to be of the dominating religion in the area
All error coins need to be seen. Take it to a coin dealer in your area.
I suggest you get a good coin book and search for your coin in there.
I will base the answer on the assumption you mean the 12-sided 50-cent coin. A first approximation will assume that the coin is round, and use half a line that connects two opposing vertices as the diameter d, and then the area is pi*d^2/4. The diameter being 31.51 mm, we get 7.79 cm2 A more accurate answer will consider the coin as a regular dodecagon, that is 12 isosceles triangles whose long side's length is the circle's radius i.e. 15.25 mm, and each have a top angle of 360/12 degrees = 30 degrees. The formula in the links gives here 3*d^2/4 = 7.4466 cm2
The area of Coin-sur-Seille is 3,310,000.0 square meters.
The area of Coin-lès-Cuvry is 6,650,000.0 square meters.
The area of Saint-Régis-du-Coin is 20.4 square kilometers.
Do a search for "Coin Dealers" in your area or, look up your local phone book.
The 1966 Australian 50 cent coin has a diameter of 31.5mm. The area of one side of the coin is 779.3 square mm.
Do a google search for coin dealers in your area or pick up a phone book.
Not enough information... You need to have your coin examined first hand ... check the web link "Coin Shows" for a coin show in your area where you can take the coin to be appraised .
Try a coin dealer in your area or, if you can adequately describe the coins, advertise them on eBay.
Find a coin dealer in your area. Most of them do buy coins.
Find a coin show in your area, take the coin, and get offers from several different dealers.
Coin dealers, coin collectors, flea markets, advertising listing in coin publications.
There are many places where you can find out how to converse coin values from U.S. to Vietnam like online coin conversing sites or local coin shops in your area.