Scientific

Quartz comes in many different forms such as clear crystals, microcrystalline rocks, and massive chunks.  Microcrystalline forms are further subdivided by transparency (chalcedony and chert) and further by color.  Each of these subdivisions has its own name.  Officially, onyx is a form of chalcedony that is characterized by black and white banding.  Different colors of banding are due to different chemical impurities often have different names such as sardonyx which exhibits reddish brown and white bands.  However, the term onyx has been applied to many different stones simply because they exhibit banding.  Examples are Mexican Onyx (travertine) and Pakistani Green Onyx (marble).  True onyx is very hard Mohs scale of 7 like quartz whereas travertine and marble are much softer and easier to scratch.  Most black onyx on the market is dyed chalcedony.

Uses

Primary use of onyx for centuries is due to its beauty, durability, and symbolic significance. It is popular for jewelry especially for amateurs because it is colorful, reasonably plentiful and inexpensive.  Additionally its hardness allows it to achieve a high polish that is not easily damaged.

Metaphysical

Onyx is a popular stone in metaphysical practices, supporting the root chakra, absorbing, repelling and transforming negative energy and grounding the wearer. It is believed to enhance strength, stamina, and self-discipline thereby promoting perseverance and supporting one's ability to face challenges and adversity.  It accomplishes this by helping to calm fears, anxiety, and overwhelming emotions and aids in decision-making by providing mental focus and discipline.