The only Third-Farthing issued during the reign of George IV was the 1827 coin.
A British 1827 copper Third-Farthing (KG IV)(minted for use in Malta), uncirculated and in absolute mint condition could fetch up to £175 GBP. If it has been circulated but still in good condition, it might fetch anything from £15 to £65 GBP.
A British 1827 copper Third-Farthing (KG IV)(minted for use in Malta)(Proof FDC), uncirculated and in absolute mint condition could fetch up to £400 GBP.
A reputable coin dealer will be able to give a more accurate valuation.
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Sorry no US silver dollar coins dated 1826
A British 1826 Sovereign, uncirculated and in absolute mint condition could fetch up to £1,550 GBP. If it has been circulated but still in good condition, it might fetch anything from £250 to £975 GBP. A British 1826 Sovereign (Proof FDC), uncirculated and in absolute mint condition could fetch up to £2,750 GBP. A reputable coin dealer will be able to give a more accurate valuation. NOTE - The reverse of these coins is upside down to the obverse. This is normal.
The 1826 British Shilling and Sixpence coins are both sterling silver and both have a lion standing on a crown on the reverse. The Shilling is 23.5 mm in diameter. The Sixpence is 19 mm in diameter.
Check that coin again. The U.S. didn't mint any $1 coins (in either gold or silver) dated 1826.
around 129 dollars
No, in 1826 it became a British colony, part of the Straights Settlements.
Assam
Please check again and post a new question. The US didn't mint any half dimes dated 1826.
No. There are two different 1826 Farthings, and each has variations. There is the First Issue 1826 British Farthing which has the year below Britannia on the reverse. Variant 1 - The standard issue coin which appears much as you would expect. Variant 2 - There is an R/E in GRATIA which is the error. There is the Second Issue 1826 British Farthing which has a wreath below Britannia on the reverse. Variant 1 - The standard issue coin which appears much as you would expect. Variant 2 - An error noted only recently, is the use of the Roman "I" instead of "1" in the date on some coins, ie. I826 instead of 1826 on the obverse. Variant 3 - There is also the Proof FDC coin.
The diameter of an 1826 British Sixpence is 19 mm. Therefore the radius (r) is 9.5 mm. The formula for calculating the circumference of a circle is 2 x Pi x r . The circumference of an 1826 British Sixpence is 59.69 mm.
100-1000+ depending on condition, accessories, etc..