Most Italian necklaces were just simple strands of pearls, sometimes with a pendant attached. Many portraits, when looked at closely, show some sort of spacer between each pearl. Most likely many of these "spacers" were knots in the stringing cord, which are still used today for both security (in case the thread broke) and to keep the pearls from rubbing on each other. However, in quite a few instances the spacers seem to be tiny metal beads, such as in the 1544-45 portrait of Eleonora of Toledo with her son Giovanni de' Medici (by Agnolo Bronzino; Galleria degli Uffizi, Florence):
Or in this one of Lucrezia de' Medici, c1560 (by Alessandro Allori; North Carolina Museum of Art):
Presumably these tiny beads served the same purpose as the knots, but with the added bonus of a subtle sparkle! Of course, with my love of sparkle, I opted for using tiny gold beads in my ACC necklace:
