Usually a ring with a seal carefully crafted upon it that an important or rich person used to authenticate a document. It was used much like a signature on a document today. The ring of kings would carry the highest authority in a land and empowered subordinates to act for the king. Examples of such rings in the Bible are: Pharaoh’s ring given to Joseph (Gen 41:42); Ahasuerus’s ring given to Haman and then to Mordecai after Haman was hanged (Esther 3:10,12; 8:2); King Darius’s sealing the den of lions after Daniel was thrown into it (Dan 6:17).
The signet could be worn on a chain around the neck. In an unusual use of the word, Zerubbabel is said to be “a signet” because the Lord had chosen him (Hag 2:23). Zerubbabel was granted Yahweh’s authority, and thus completion of the temple was guaranteed. Another unusual use of the word was the special engraving of the stones on the ephod of the high priest “as a gem cutter engraves a seal” (Exod 28:11 HCSB)