ONYX
Onyx: Banded Chalcedony, Black Onyx & Identification
Onyx is a variety of chalcedony, a microcrystalline form of quartz, traditionally characterized by parallel bands. In modern jewelry, the name black onyx often refers to uniformly black chalcedony, which may be dyed. Distinguishing natural color from treatment can matter to collectors and buyers.
Table of Contents
Formation and Geological Setting
Banded onyx forms as silica-rich solutions deposit layers in cavities and fractures. The term is also used in geology for banded calcite or aragonite materials sometimes called cave onyx or Mexican onyx, so context matters.
Properties and Appearance
Quartz-based onyx has Mohs hardness around 6.5–7 and a waxy to vitreous luster. Traditional banded onyx has parallel layers, while commercial black onyx may be uniformly dark. Some pieces are dyed to strengthen color.
Identification and Collecting
Onyx can be confused with other black stones, glass, and dyed materials. Ask for disclosure of treatments and use appropriate gem testing when authenticity matters. The name can refer to different materials in different trade contexts.
Care and Handling
Clean chalcedony onyx with mild soap, lukewarm water, and a soft cloth. Avoid harsh chemicals and prolonged heat, especially if dyed. Protect polished surfaces from scratches and store separately.
Frequently Asked Questions
Can appearance alone confirm identity?
No. Similar materials and treatments can look alike. Use credible documentation and appropriate gemological testing when certainty matters.
What should collectors keep?
Retain locality details, seller disclosures, treatment information, and any laboratory reports with the specimen.
Further Reading & Sources
- Mindat — mineral data and localities.
- Handbook of Mineralogy — technical descriptions.
- Gemological Institute of America — gem identification and education.
- Wikipedia: Onyx — background and references.