There is no such coin.
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There are a lot of misunderstandings about this coin ! It has nothing to do with mother Teresa - of course, it is : Maria Theresia, Empress of Austria. The problem with this coin is, that there are about 800.000.000 coins struck in the 20th century, but with the original date 1780 ! Value : only 20 dollars...
$18 is the value of a 1780 x countermark coin 20 piastres.
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The value of a 1780 Maria Theresa Thaler can vary depending on its condition and any unique characteristics it may have. Generally, these coins can range in value from about $20 to several hundred dollars, with rare or well-preserved pieces commanding higher prices. It's best to consult with a numismatic expert or use a coin appraisal service to determine the specific value of your coin.
I'm guessing you're referring to a Maria Teresa coin. See the Related Question for more details.
Found this answer on another site "Your coin is a commemorative coin, issued by the British Overseas territory of Turks and Caicos, a group of islands in the West-Indies. I cannot find any mintage for this coin, but the Standard Catalog of World Coins attributes this coin (Turks and Caicos 5 crowns KM#225) with a catalog value of 10 US$ in uncirculated condition."
The value of a 180 einen thaler coin can vary depending on its condition, rarity, and historical significance. It is recommended to consult with a numismatic expert or use online resources such as coin pricing guides to determine its specific value.
The Maria Theresa thaler coin was first minted in 1741 but after 1780, has since been dated to 1780. The coin was once accepted as a form of global currency, but is today seen as a collectors' item instead.
Wish I had logged in for the other coin questions. You like eceryone else need to describe the coin much better. In your case it is at least somewhat understandable but if you got as far as knowing it was Maria Theresa why not tell us the origin. The coin was produced in several countries in several provincial mints. It could be worth as little as $5 and as much as $200,000. You need a pro and in reality maybe 2 if the first one says it is worth something
The value of a Theresia silver coin can vary depending on its condition, rarity, and current market demand. It is recommended to have the coin appraised by a professional numismatist or check online auctions or coin dealers for current pricing information.
First, it is highly unlikely your coin was struck in 1780, the Maria Theresa thaler has been struck in large quantities from 1780 all the way to the present day. If you are really interested, there are ways to determine by the die if yours is an older strike or a newer one. But generally, these coins sell for silver scrap, or perhaps a bit more. You should easily be able to sell it for $20-25 depending on the price of silver.