Polished Black Onyx Gemstone

A beautifully polished 'black onyx' cabochon.

This piece exhibits a vitreous lustre so the surface interacts with light in a similar way to glass.  It has a glass-like appearance and texture which makes it incredibly tactile.

Upon looking at this stone you would be forgiven for thinking it's black onyx.

Black onyx does occur naturally but is relatively rare so the vast majority is dyed agate, chalcedony or calcite.

With it having visible horizontal bands in the lower section it could be agate. I think that's unlikely because the banding in agate tends to be curved.

Natural onyx exhibits bands that run parallel to each other (as opposed to being curved). Therefore this gemstone could be natural onyx that's been dyed but I believe it's more likely to be calcite.

Calcite is more readily available than onyx which means it's less expensive. It's also softer which makes it easier to work with. 

The only way to know for certain would be to do a scratch test or to add a drop of acid to the stone.  Unfortunately both would cause some level of destruction.

This gemstone is sold sold as onyx even though that's unlikely to be what it is.

As well as being beautifully polished it's translucent between some of the layers.  It's a lovely gemstone which is the reason we chose it for our collection.