Years ago, I bought out a lot of unmounted Italian cameos, lava cameos, micro mosaics and gems from the estate of an old goldsmith (family business for a couple centuries) in Paris. This one, never mounted, was among those. I'm working through that collection now. Each sold separately. Years ago we did a lot more with micro mosaics than we do now. We sold a whole lot over the decades to the dealers in Rome and Florence, then would see them in their shop windows at triple value when in those cities. This is a beautiful and large one at 40mm x 20.5mm, rectangular which is a bit more rare than round or ovals.
While this one has tiny sand-sized tesserae and is near=pristine, it is a collector's piece, or for someone who wishes to have a goldsmith make a suitable gold mount for it. I've seen these worked up into a stunning necklace with a variety of Grand Tour (c.1800-1850s) micro mosaics - truly to die for. Measurements noted on the photos: 40mm x 20.5mm and 4mm thick. No serious chips (one tiny nip to a corner which would be covered by a mounting, no losses to tesserae, nearly pristine at 170 years old or more.