De Artibus Romanorum IV
The gemstones onxy and sardonyx often have veins of white onyx running through them. The so-called state portraits cameos, commissioned by or for the Roman emperors, ingeniously exploit that flaw in the stone.
Following Hellenistic models (the most famous being the Tazza Farnese), the carvers cleaved the stone at a vein and working on the cut surface, remove the white stone to varying depths around the image, to create low-relief white portraits set against dark backgrounds. Given the hardness of the stone and the thinness of the vein, and the iron tools the degree of refinement, modeling, and detail is extraordinary.
The cameo technique was imitated in luxury Roman glassware, like the Portland Vase, and, much later, in the Neo-classsical jasperware made by Josiah Wedgewood.
Intaglio is the opposite process, wherein the image rather than the background is carved down into the stone. The Romans excelled at this technique as well.