Cameos: Shell, Hardstone, Lava and the Ones That Are Plastic
Every family has one, and most of them are shell. Telling shell from agate from a moulded resin takes a lamp, a loupe and about a minute, and it changes the value by a factor of ten.
A cameo is a carving in relief, the design standing proud of its background, and the whole family is defined by that one fact. Its opposite, the design cut into the surface, is the intaglio, and the cross-section diagram there settles the distinction permanently. What varies inside the family is the material, and the material is where the value lives.