Personal seals in the form of engraved gems, which had been used for centuries in Greece and Rome, were at first treated with apprehension by the early Christians. Writing around 200 ce, Clement of Alexandria cautioned that the devices should be consistent with Christian teachings. The earliest identifiable Christian gems date from the second quarter of the third century and were produced primarily in Syria and Asia Minor. Many of these seals bear inscriptions in Greek naming Jesus, the chi-rho monogram, or the Christian acrostic ΙΧΘΥC. Other examples displayed the image of two fish flanking an anchor or the Good Shepherd. Old Testament images of the sort seen in the catacomb paintings in Rome, such as Noah, the Sacrifice of Isaac, Daniel, and Jonah, were also used. Later (5th–7th century) Byzantine gems, primarily from Constantinople, reflect the Christian pictorial cycles of the time.