An extremely rare and unusual bullet coin from Siam is being offered as part of Morton & Eden's June 5 auction in London.

This example would not have been put into circulation, instead being used for ceremonial purposes

The bullet coin, which is around the size and shape of a marble, will be sold with a £6,000 ($9,098) estimate. They were in use in ancient Siam (now Thailand) until 1860, when traditional flat coinage was introduced.

The example at auction had the unusually high value of 20 baht, but was never in circulation as it was made for ceremonial purposes in 1880.

This example of the silver coin was introduced to commemorate the death of the mother of Rama V (1853-1910).

Rama V was known as the Royal Buddha and is considered one of the country's greatest rulers.

Made from bars of silver and gold, the bullet coins are thickest in the middle, and are bent round to form a complete circle. The shape is thought to be inspired by that of cowry shells, which were widely used as money in ancient times.

"They are always struck with countermarks to prove their authenticity, one of which is always an ornate chakra wheel," commented Morton & Eden's Jeremy Cheek.

"This example also has an ornate royal crown mark and an exceptionally rare mark showing a Thai flower with the date 1242."

View our full range of rare coins for sale, starring rare Italian gold pieces and English silver coinage.