A signet is a small seal, especially one used to mark documents with a special mark.
A signet can make people think of important papers, old letters, or secret messages.
Examples:
The king's signet was used to mark the letter.
I found a signet in the old box.
The signet was made of gold.
The signet was lost in the sand.
The signet was as big as a house!
The signet was very old and very special.