Onyx is a banded chalcedony in which the predominating bands are black. Bands in sardonyx are dark red rather than black. Orange-red chalcedony is known as carnelian.
Sardonyx is the gemstone from which cameos are made. Especially in ancient times, sardonyx cameos were cut either as intaglio, with images carved into the stone, or more commonly as relief, with background stone removed to leave an image.
A profile or portrait accounted for most of the images on ancient cameos. Current cameo portraits portray more modern profiles and can include florals and other objects.
In all cases, carving a cameo from sardonyx occurs at the interface of two layers. Most often, the dark red layer remains as background while white layers of the rock contain the image. Some ancient sardonyx stone cameos have a brown layer atop the white layers of the relief image. Precision craftsmanship in carved sardonyx is manifest in the incredible detail expressed in such cameos.
Historical examples of ancient sardonyx cameos are on exhibit at Bowers Museum in Santa Ana, California. The collection is called Gems of the Medici after the famed Italian family that funded and collected the works of the artisans of Florence over several generations.
Current examples of cameos said to be sardonyx are made of shell rather than chalcedony stone. Sardonyx shell cameos originate from shell, whereas sardonyx stone cameos originate from chalcedony stone. The two can be difficult to distinguish in a cameo by casual observation.
Discover the difference by the descriptor in the name or the currency of the creation. Cameos made in modern times can be taken as sardonyx shell with older, if not ancient, cameos assumed to be made from sardonyx stone.
More often than not, current sardonyx designs appear more commonly as large, smooth-surfaced cabochons for pendants, without any intaglio or relief images carved from the stone at all. The starting material is sardonyx agate, a form more common than the banded varieties of sardonyx stone.