Onyx is a variety of AGATE, but instead of curved bands it has straight parallel ones of alternate black and white. If reddish brown bands occur instead of black, it is called SARDONYX. The onyx name is generally applied in commercial dealings to the black bands, and this has led to any black chalcedony being called onyx, whether it is natural or dyed. Baja California, northwest Mexico, was a major source until cheap imitation stones appeared on the market.
The ancient Romans carved different patterns in each layer of a multi-layered onyx and often used the stone for their seals, carving a design in negative relief to produce the raised print. They had first used the onyx name for a variety of marble having white and yellow veins, and onyx is the Greek word for ‘claw’ or ‘fingernail’, because these veins resemble the colours of a fingernail. The marble is still called ‘onyx marble’, being less valuable and softer than onyx.
Onyx has been dyed since ancient times to enhance the colour. It has traditionally been used for carving cameo brooches. Because of the stone's different bands, part of the black layer can be carved away to create a white background while leaving a black design on top. The colours can be reversed: Wedgewood ceramic ware, for instance, resembles onyx cameos with white figures over a dark grey background. Onyx items are affordable, and include brooches, beads, earrings, necklaces, pendants, stones for rings, chess pieces, bookends, cups, vases, and figurines such as animals. The stone may scratch or chip easily. Natural black onyx is rare, so the commercial black variety is nearly always agate that has been stained by the sugar-sulphuric acid treatment, soaked in a sugar solution and then heated in sulphuric acid to carbonize the sugar.