Black onyx is not a distinct chemical group but rather a variety of chalcedony, which is a cryptocrystalline form of silica (SiO2).

The black color of onyx is due to the presence of impurities, often iron oxides or manganese oxides.

So, while onyx itself isn't a chemical group, it's chemically classified as silica dioxide with the added presence of trace amounts of iron or manganese.