Cameo glass
Cameo glass is made of two or more layers of different colours, usually a pale layer over a dark one, with the top layer carved, etched or ground away so that the design stands out in relief. The effect is like a hard-stone cameo. The best-known ancient example is the Portland Vase, a Roman piece made in the first century.
In the nineteenth century, cameo glass was revived in England and France, and the layers were cut with wheels and by hydrofluoric acid. The same idea is used in flashed architectural glass, where a thin coloured layer over clear glass is removed by sandblasting or acid to form an image.
It is one of the more demanding forms of glass decoration, because every area removed is permanent.