A SUPERB BELLE EPOQUE OPAL PENDANT NECKLACE   Centering upon a heart-shaped opal pendant, within a calibré-cut sapphire, old European-cut diamond &amp; blue enamelled frame, enhanced by sculpted scrolling gold iris motifs, suspended by twin diamond collet lines from a similarly-set fan-shaped surmount, to the backchain of alternating diamond collets &amp; navette-shaped blue enamelled foliate motif links, joined to a smaller heart-shaped opal plaque within a sculpted gold iris motif frame, circa…