What is an onyx in mineralogy ?
Onyx : definition
Onyx is mineralogically a variety of chalcedony, characterized by a colored and regular banding, giving it a certain aesthetic and decorative value. In the ornamental stone industry, the meaning of the term "onyx" has extended to any rock presenting a colored and regular banding, liable to take a beautiful polish : it is then often limestone or marble, and not no chalcedony.