Black Onyx

Onyx is agate and the same thing and
Black Onyx does have alternate stripes of different

Yes, they are both subcategories of chalcedony. Agate is defined as having concentric bands, while the onyx flat bands superimposed. However, for many black onyx is dyed in any event, most rock shops will be sold as "black onyx", even if it is not banded. Onyx True is expected in black and white stripe, but in practice all kinds of chalcedony get labeled as onyx.

Here's what MinDate has to say about Onyx:
http://www.mindat.org/min-2999.html
Agate
\ Http: / / www.mindat.org/min-51.html
Chalcedony:
http://www.mindat.org/min-51.html

Neither chalcedony, agate, onyx or regarded as a mineral, because they are a mixture of quartz and moganite.

The form of calcite is actually called onyx marble (and is illegal to possess or inappropriate in some states in the U.S. because he just cave formations).
http://www.mindat.org/min-3000.html

Black Onyx is as you say, black agate, while oynx is alternating layers and white onyx black.Sard is layers of white and orange (carnelian) or brown (SARD). There is another variety called multicolored onyx Mexican onyx this, I believe, not an agate, but a form of calcite and alabaster AR is much softer.

... Yep! Says Ken. And often Onyx Retail is the lightest variety dyed black for aesthetic reasons. Metaphysically, they have different properties and require a special discernment to distinguish.