Onyx vs Agate
This is a question from Ian:
"How about a black agate/black banded agate? is it onyx? same qualities?..thank you"
Here is the deal.
Agate and onyx are similar because they are both quartz stones, and they are both from the chalcedony family.
Both agate and onyx can have bands. Agate is considered the banded variety of chalcedony. It has circular bands, whereas the bands found on onyx are more or less straight and parallel.
Now, black onyx usually is not uniform in color. It has white areas which are treated to make it black. Black agate is also dyed and often sold as black onyx.