'Signet' in the Bible

So the king removed his signet ring from his hand and gave it to Haman the son of Hammedatha, the Agagite, who was hostile toward the Jews.

So the royal scribes were summoned in the first month, on the thirteenth day of the month. Everything Haman commanded was written to the king's satraps and governors who were in every province and to the officials of every people, province by province according to its script and people by people according to its language. In the name of King Ahasuerus it was written and sealed with the king's signet ring.

The king then removed his signet ring (the very one he had taken back from Haman) and gave it to Mordecai. And Esther designated Mordecai to be in charge of Haman's estate.

Now you write in the king's name whatever in your opinion is appropriate concerning the Jews and seal it with the king's signet ring. Any decree that is written in the king's name and sealed with the king's signet ring cannot be rescinded.

Mordecai wrote in the name of King Ahasuerus and sealed it with the king's signet ring. He then sent letters by couriers on horses, who rode royal horses that were very swift.