The Keramion is a ceramic tile with an image of Jesus on it and is closely associated with the Mandylion. It has its own legend surrounding it and it is believed to have been displayed at the Edessa gate.

He also released an order and law in writing, that whoever entered that gate of the city, should first ascribe reverence and veneration towards the wonderworking and honorable icon of the Lord, and then enter the city. This order and law of Abgar was kept until the end of his life and that of his son. When his grandson became the recipient of his paternal inheritance, he turned away from piety and voluntarily turned to the religion of idols. Wherefore he wanted to place over the gate of Edessa the image of a demon, and take down the icon of Christ. When the Bishop of Edessa found out about this by divine revelation, he showed the proper care and attention. Because the place above the gate was deep, made like a rotunda with a cylindrical roof, he lit a lamp before the holy icon of Christ, and in front of it he placed a ceramic tile (keramion), which he covered over with bricks and asbestos, thus closing off that place, making that section of the wall look even. Therefore, since the icon of the Lord could not be seen, the plans of the impious one were halted, and the holy icon was not taken down.

Many years passed, to the extent that the place where the holy icon was located became forgotten. When the king of the Persians Khosrow, during the reign of Heracleus the emperor of the Romans (610-641), fought against the cities of Asia Minor in 615, he arrived in Edessa. Having come against it with every instrument in his arsenal, he brought fear and anxiety to its citizens, who took refuge in God, and begged Him with tears so they could quickly find salvation. One night a glorious woman appeared to Bishop Eulavios, who told him that he would do much good if he took the icon of Christ made without hands that was hidden above the gate of the city, showing the location with her hand. The Bishop went to the spot and began digging until – O the wonder! – he found the divine icon of the Lord, whole and complete, and the lamp he still found to be lit after more than five hundred years. And the ceramic tile, which the Bishop then had placed before the Holy Mandylion, on this same ceramic tile he found imprinted another icon of the Lord, precisely similar to the Holy Mandylion. When the citizens of Edessa saw these two divine imprints and icons of the Lord, they were filled with spiritual gladness and rejoicing. … -holy.html

Leo the Deacon (born ca. 950) writes that when
Nikephoros II Phokas (r. 963–69) took the city of Hierapolis in 966, he captured the Keramion,
an acheiropoieton and tile that was miraculously imprinted with an image of Christ. … tation.pdf

Reports suggest the Holy Keramion was brought to Hierapolis, where it was obtained by Emperor Nikephoros Phokas in October 966 before he besieged Antioch, and then brought to Constantinople on 24 January 967. Other reports suggest these were two different Tiles, with the one in Hierapolis originating from the time of Christ when Ananias was returning to Edessa with the Mandylion and he placed it in between two baked tiles, thus creating the Keramion that way. One tradition also states that the Ancha icon of the Savior in Georgia, which was brought there to escape Iconoclasm, is in fact the Holy Keramion from Edessa. Phokas had the Holy Keramion from Hierapolis placed in the Blacharnae Church in a golden box ornamented with precious stones and later had it deposited in the Church of All Saints. In the eleventh century it joined the Holy Mandylion in the Pharos chapel where it was displayed in a golden capsula suspended from the ceiling on silver chains. When Constantinople was overrun by the Crusaders in 1204, the Holy Keramion became lost to history.
This ceramic tile, known as the Holy Keramion (or Keramidion), is honored by the Church on August 16th, together with the Holy Mandylion. … nce-abgar/