Black onyx is a variety of chalcedony, a microcrystalline form of quartz (silicon dioxide), that displays solid black coloration and has been one of the most popular and recognizable gemstones throughout human history. This cryptocrystalline quartz forms in volcanic rock cavities and is technically a form of agate, though it lacks the banded patterns typical of most agates, instead presenting a uniform black appearance that can range from deep matte black to lustrous jet black when properly polished. Much of the black onyx available today is enhanced through dyeing treatments of naturally gray or banded chalcedony to achieve the intense, uniform black color that is most desired in jewelry and decorative applications.
- Color range: Solid black from matte charcoal to deep jet black; some natural specimens may show very subtle gray banding or variations
- Sizes: Available from small accent stones (4-5mm) to very large specimens (several inches); excellent availability across all size ranges
- Shapes: Excellent for all applications including cabochons, beads, carved pieces, and cameos; takes exceptional polish for smooth, lustrous finish
- Origin: Worldwide including Brazil, India, Uruguay, and Madagascar; most commercial material comes from Brazil and India
- Hardness: 6.5 to 7 on Mohs scale, good durability for jewelry applications with reasonable care and protective settings
- Treatments: Commonly dyed to achieve uniform black color; natural solid black onyx is relatively rare in nature
- Polish quality: Takes exceptional high polish creating mirror-like surfaces; one of the best stones for achieving lustrous black finish
- Carving properties: Excellent material for detailed carving work including cameos, intaglios, and decorative objects
