"The Christian tradition of 'treasure' bindings, covered with gold and silver, ivories, enamelwork, and gems, had its origin in late Antiquity and continued unbroken for a millennium. The earliest reference to such bindings in a Christian context is found in a letter of St. Jerome, dated 384, where he writes scornfully of the wealthy Christian women whose books are written in gold on purple vellum, and clothed with gems. It is noteworthy that he specifically associates jewelled bindings with purple codices, for a dozen or more such biblical manuscripts of the fifth and sixth centuries have survived. None is any longer in its first binding, but we have a clue here to the external treatment originally given to these luxurious volumes. . . ." (Needham, Twelve Centuries of Bookbindings 400-1600  21).
"From the time of Constantine's decree, Christian book production was in a position to develop freely, but already in Diocletian's time Latin biblical manuscripts must have been available in large numbers. A century later Jerome became impassioned about conspicuous luxury in Christian books. He wrote with biting sarcasm about biblical codices of old, badly translated texts: 'veteres libros vel in membranis purpureis auro argentoque descriptos, vel uncialibus, ut vulgo aiunt, literis onera magis exarata quam codices', i.e. manuscripts made with expensive material and with 'inch-high' letters. He compared this with his own ideal: 'pauperes scidulas et non tam pulchros codices quam emendatos', and one can refer immediately to the plain St Gall gospel manuscript (Σ) saec. V, which stands very close to the text-critic Jerome" (Bischoff, Latin Palaeography: Antiquity and Middle Ages  184.)
The manuscript to which Bischoff refers is Codex Sangallensis 1395, the earliest surviving copy of the Vulgate gospels.