In that day, saith the LORD of hosts, will I take thee, O Zerubbabel, my servant, the son of Shealtiel, saith the LORD, and will make thee as a signet: for I have chosen thee, saith the LORD of hosts. Haggai 2:23
The meaning is evident from the importance of the signet ring in the eyes of the Oriental, who is accustomed if he owns a ring to carry it constantly with him, and to care for it as one of his most prized and valuable possessions.
The signet being an emblem of authority, was used to stamp documents and other legal articles. Indeed, without being stamped by a signet, no document was considered authentic.
(Bowen, Barbara M., Strange Scriptures that Perplex the Western Mind, WM B. Eerdmans Publishing Co., Grand Rapids, Michigan, 1944, pg 58)