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Citrine, a natural yellow variety of quartz (amethyst is the purple variety) is far more uncommon than shell, or a striated pebble you might pick up on the beach, from which cameos are usually carved. It is hard, and difficult to carve, being brittle, so one rarely sees citrine cameos. This nicely carved dog's head is inset with pearl eyes, in a 15ct gold mount, and dates from around 1870.
It is approximately 3/4 inch:20mm in diameter with a long pin. It could be converted into a brooch or pendant, if required.