The vast majority of material being sold as the mineral 'onyx' is not onyx at all.
Onyx is a black and white banded variety of agate. Onyx and agate are both types of chalcedony.
The confusion over the precise definition comes from geological variations, overlapping terminology, subjective classification and evolving scientific understanding of this mineral.
The stones we currently have in our collection are primarily calcite.
They are however marketed as 'onyx'.
Banded calcite is often used instead of onyx because it's more readily available. It's also much softer so is easier to work with.
OBSIDIAN
