The onyx are varieties of chalcedony.
Mineralogists describe them as varieties of agate. The name comes from a Greek word meaning “nail” because of the color of the loose nail.
The gemologists and gem cutters talk about completely black gems or in banded layers of black and white colors due to be engraved or carved . They are ranked: the genuine onyx or “Arab onyx”, a black onyx or agate made of a black to very dark brown layer covered with a white layer, and almost flat, the sardonyx, with alternating white and black colors, or again brown, in which were carved famous cameos with two, three or more layers, such as the grand cameo of Sainte-Chapelle, the cameo of Vienna … The carnelian-onyx, with a red layer, used as a base, topped with a white layer, the Nicolo with a very thin top layer so that due to dispersion and transmission, the black brilliance of the underlying layer appear in bluish tones. The limestone onyx or “marble-onyx” or “onyx-marble,” is just aragonite or white calcite, yellow, brown or greenish, still zoned. It is formed by deposits of hot springs or stalactiforme concretions.