Some fragments of another vase of a similar character were found at Pompeii at a later date; the pieces after passing through various hands are now in the British Museum, where they have been united to form (with extensive gaps) an _œnochoë_ or jug, known as the Auldjo vase, from the former owner of most of the fragments; in this case the decoration of the parts preserved consists chiefly of vine and ivy leaves. There are at Naples many fragments of onyx glass equal in beauty and skill of execution to these well-known vases. Among these, the half of a patera decorated, on a dark blue ground, with a mask surrounded by the leaves of the Oriental plane, is of exceptional merit. In other cases the parts in relief seem to have been cast separately and fixed on to the surface, a technical process of quite another nature. In all these examples the work of the artist follows closely on the lines of the carver of cameos—especially of those cameos where advantage is taken of the parallel layers of the natural stone, as in the case of the sardonyx and of the niccolo; it is for this reason that I have described the material of our Barberini and similar vases as onyx glass. But there was another and purer variety of quartz that was coming more and more into favour during the third and fourth centuries. From this time onward all through the early Middle Ages, if we are to judge from the treasures preserved in Christian churches, to nothing was more value attached than to vases and cups of rock crystal, often of imposing dimensions, carved in shallow or deep relief. When once the process of making a clear colourless glass was mastered, this natural crystal could be very closely imitated in a material which was more easily worked. The carvings on the great majority of the examples of rock crystal that have come down to us—for example, the vases in the Louvre from the Abbey of St. Denis, and those still preserved in the treasury of St. Mark’s—are of a distinctly Byzantine, if not rather of a Sassanian or even Saracenic character, and this style is reflected upon much of the ‘crystal’ glass which is so often confused with the harder stone.[40] The Romans of the fourth century were great masters of the art of cutting hard stones. Along with a general decline in taste and artistic invention, there was some advance in the direction of what we should now call applied science, and this is exemplified in the nature of the ‘metal’ and in the method of carving of the later Roman glass.