Shell Cameo surrounded by diamonds and enamel.
This is a “Family Order” from Queen Victoria. The British sovereign bestows these decorations to female members of the royal family. The pendant is a depiction of the Sovereign ( in this case, Queen Victoria and Prince Albert as well, in profile.) Surrounded by diamond brilliants and surmounted by a diamond and enamel crown, the decoration is worn on formal occasions over the heart. It is suspended from silk moire ribbon, the color of which changes for each successive reign.
Since the reign of King George V the order has changed from a shell cameo profile depiction to a facial portrait in color on an ivory disc covered with crystal.