A rustic style cast iron coaster in the shape of an ancient Japanese coin called Wadoukaichin 和同開珎 . Traditionally, Wadoukaichin was made from copper or silver in 8th century Japan. It is the oldest official Japanese coinage, minted on the orders of Empress Gemmei.
"Wadoukaichin" is the transliteration of the four characters in the coin's inscription, which is thought to be composed of the era name Wadou (和銅, "Japanese copper"), which could alternatively mean "happiness", and kaichin, thought to be related to "currency".
Coaster is 8.4cm in diameter at widest point. 7cm internal diameter / 1cm height