Onyx is a name that is correctly applied to single coloured layer of Agate, such as Black Onyx. It is also correct to call banded agate Onyx, when it occurs in a Black base and White upper layer, such as in a Cameo.
If the base material is brown, it is called Sard-Onyx.If the layers are based on red, Cornelian-Onyx. If the layers are more numerous, or in any other pattern or colours, it is called Banded Agate.
Onyx has been, and is, carved into “hard rock Cameos” by Glyptographers or “stone Engravers”. The most famous area for this kind of work is Idar Oberstein where family crests boasting minute details have been produced for centuries. The process was all done by hand and is a closely guarded secret. Even today, with computer aided designing machinery, it is very difficult to reach the same level of quality as they routinely produce in the products from this area.
Regular, smooth Black Onyx has been used for jewellery for a long time. It is durable, fairly tough and hard; many men sport signet rings, tie bars and cufflinks set with this stone. Dramatic, as well as everyday, pieces abound for ladies as well, for as we all know, black goes with everything.
Onyx is often confused with Onyx Marble which, in fact, is a Calcite or Calcite/Aragonite metamorphic rock. The name derives from the Greek where onyx roughly translates to “fingernail”, referring to its translucency. This material is often crafted into carvings, like primitive Chess sets, by locals in Mexico and the South Americas for the tourist trade. It is soft, fragile and fractures easily. It is sensitive to heat and mild acids and also separates at the banding with little effort. It is not really suitable for jewellery except as “native” wear.