Nicolo is a cameo or intaglio that is carved in onyx in such a way that the image displays a translucent bluish/grey color. This thin top layer of design lays atop a thicker layer of black material that shows through, imbuing the design with the characteristic tint of blue. Nicolo pieces have been carved since antiquity.

Egyptian Nicolo Intaglio, second century AD. Image courtesy of The Metropolitan Museum of Art.