Amber

There was a chamber decorated with six tonnes of amber, known as the Amber Room.

An excerpt from the article 18 facts about amber
Created for Frederick I of Prussia at the beginning of the 18th century, it was given as a token of friendship with Peter the Great and placed near St. Petersburg. During World War II, it was ransacked by Nazis and said to be brought to Germany. It has been reconstructed by 2003, however, presently it is an allegory of a lost treasure.