In 1955 Hungary became the first country in the world to issue a “metal” stamp. And below you see an example. Aluminium foil glued to paper was used in the production process.
Since then there have been many stamps released incorporating the use of the material. below is a 2001 sterling silver foil stamp from the Netherlands.
Dutch 17th Century national hero Piet Hein was commemorated in 2011 with this silver foil stamp.
Pope John Paul II had visited the Solomon Islands in 1984. After his death in 2005, they issued this commemorative set of silver foil stamps.
Pope John Paul II visited Gambia in 1992 and also commemorated his life in 2006 by producing the “metal” stamps below.
And the top value stamp in the “Marianne des Français” definitive set from France in 2006 was silver foil coated.
Kyrgyzstan also issued the silver foil stamps below in 2005, marking the 60th anniversary of the end of World War II and featuring prominent WWII heads of state and military leaders.
Many, many more stamps have been produced utilising various materials – maybe you have some interesting examples in your collection.