I have stitched circles all over both of the front pieces in metallic thread using feather stitch. However there was something missing so I painted pearl beads and stitched them through the border. This has brought the embroidery to life.
The bolero will close with a hook and eye. The scallop shell represents a central bow, with the border as a trailing ribbon.
This second sketch is still complicated and fussy, but I liked the idea of layered strips to represent the years of the growth of the shell.

This sample is very delicate but is still over complicated and doesn't marry well with the simplicity of the stitching on the bolero. It was worth trying, though.
I have decided to create a scallop shell that will be covered in feather stitch in graduated colours, with lines converging across the shell. The background fabric will be the same colour as the main jacket, without the machine embroidery. It will be slightly padded after stitching and will probably have converging quilted lines, mimicking the real thing. The colours will be the reverse of the stitching shown here, so that it will be dark at the outer edge and lighter as the lines converge.