Most commercial-grade stones sold as 'onyx' are not onyx at all.
Onyx is a black-and-white banded variety agate, but it can also be a single-coloured agate with alternating shades of light and darker-coloured bands.
Onyx and agate are both types of chalcedony, which is a variety of quartz.
The confusion over the precise definition of onyx comes from geological variations, overlapping terminology, subjective classification and evolving scientific understanding of this mineral.
The small number of 'onyx' stones we currently have available are calcite, however, they're widely marketed as onyx.
Banded calcite is often used instead of onyx because it's more readily available and softer, which makes it easier to work with.
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