Hastings' Dictionary of the Bible (1898)
SEAL. 1 Kgs 21:8. This was usually employed to authenticate public or private papers. Jer 32:10. If a door or box was to be sealed, it was first fastened with some ligament, upon which clay or wax was spread and then impressed with a seal or signet. Frequently a ring with some inscription on it was used as a seal, by the delivery or transfer of which the highest offices of the kingdom were bestowed. Gen 41:42; Esth 3:10. In sealing the sepulchre.
Matt 27:66, it is probable that the fastening of the stone which secured the entrance was covered with clay or wax, and so impressed with a public or private seal that any violation of it could be discovered at once. See Rings, Letters. Phoenician Seal. Modern travellers describe the seal used in the East, at the present day, as made of cornelian, or agate, with the name or title of the writer, or some verse of the Koran or other motto, engraved upon it. 2 Tim 2:19. It is fastened into a ring and worn on the hand. Song 8:6.
When used it is either applied to the wax, or is covered with some substance which, being stamped on the paper, leaves the desired impression. The word "seal" is used figuratively in the Bible to denote an act or token or process of confirmation. Rom 4:11; Eph 4:30.