In 2009 to celebrate the 250th Anniversary of Kew Gardens, the Royal Mint released a very Limited Edition Kew Gardens Piedfort 50p Fifty Pence Silver Proof Coin struck in solid .925 Sterling Silver.

Piedforts are coins which have been specially struck on thicker than normal blanks.
As their name suggests they are closely associated with France, where from the twelfth century they were apparently issued by the Kings as presentation pieces.
In Britain the practise of striking piedforts is later and less frequent, but medieval and Tudor examples are known.