Head of the Saviour

Can any of your readers give me some information about an engraving of our Saviour, which may just now be seen in many of the London print-shops? It represents the side-face, and is said to be a fac-simile of a likeness engraved on an emerald by order of some Roman Emperor, and which served as the ransom of some other famous person (who, I quite forget). Is this really the truth?