Onyx is a familiar gemstone but not many people realise that it belongs to the quartz family. It is a variety of chalcedony, specifically agate, as it has contrasting bands of colour, usually white and black or brown. The bands are usually straight bands.
We are used to seeing onyx as a black stone but this is because the stone has been dyed to hide the banding. Dyeing and staining of onyx (and other coloured agates) is not a modern activity. Pliny, in his books titled ‘Natural History’ written in the 1st century AD, talked about changing the colour of gemstones by boiling them continuously in honey for a week and written accounts from the 19th century discuss boiling pale agate in a sugar syrup and then treating it with sulphuric acid to turn it black.
Onyx was used historically for intaglios and despite its natural banding, it was often dyed to create clearer white layers. The stone was first dyed all black and then one side was placed in acid to bleach it white. Then the engraver carved out the design, exposing the black underneath and leaving a white image on top.
As a black stone, in the Victorian era, onyx was used in signet rings, mourning jewellery and for facetted beads necklaces. Today, it is commonly seen in signet rings, the onyx set with a central diamond or pearl.