In ancient times a signet ring functioned much like a personal signature does today. A king or other high official would stamp an official document with the symbol on his ring to establish its legality and show that it was issued under his authority.
God compared Zerubbabel, the Jewish governor of Jerusalem, to a signet ring. He had invested Zerubbabel with the highest honor and would use him as His representative to bring about His purposes in the city of Jerusalem.
When the pharaoh of Egypt appointed Joseph to a high position in his administration, he gave Joseph his signet ring to show that he had the authority to act on his behalf (Gen 41:42).
HAGGAI 2:23 In that day. . .will I [God] take thee, O Zerubbabel. . .and will make thee as a signet [signet ring. NIV]: for I have chosen thee, saith the LORD of hosts.