An anaglyph on a precious stone. The anaglyph is when the figure is raised in relief; an intaglio is when the figure is hollowed out. The word cameo means an onyx, and the most famous cameo in the world is the onyx containing the apotheosis of Augustus. These precious stones have two layers of different colours, one serving for the figure, and the other for the ground.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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