In 2015 to celebrate Queen Elizabeth II the Longest Reigning Monarch, the Royal Mint on behalf of Great Britain released a very Limited Issue Longest Reigning Monarch Piedfort £5 Five Pound Silver Proof Coin struck in solid .925 Sterling Silver.

These legal tender coins have been struck to proof quality using specially prepared dies and highly polished blanks.

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.