Judging quality
Sardonyx and natural banded onyx show honest layers; solid jet-black material is essentially always treated. Look for smooth, even polish with no dye pooling at fractures.
Chalcedony
Onyx
Chalcedony with straight parallel banding. Most 'black onyx' on the market is grey chalcedony dyed black, a practice so old and so standard that it is not considered a deception.
Care
Durable; avoid harsh solvents that can lift dye on treated pieces.
History & lore
Roman cameo carvers used banded onyx to cut white figures against dark ground — the technique behind the Gemma Augustea.
Cultural history, recorded as tradition rather than as medical or scientific claim.
Common questions about Onyx
- What is Onyx?
- Chalcedony with straight parallel banding. Most 'black onyx' on the market is grey chalcedony dyed black, a practice so old and so standard that it is not considered a deception.
- How hard is Onyx and what is it made of?
- Onyx is a Chalcedony-family mineral with the composition SiO2. It crystallizes in the trigonal system and rates 6.5-7 on the Mohs hardness scale. It occurs in black, white, banded.
- Where does Onyx come from?
- Notable localities for Onyx include Brazil, India, Uruguay.
- How do you tell good Onyx from filler?
- Sardonyx and natural banded onyx show honest layers; solid jet-black material is essentially always treated. Look for smooth, even polish with no dye pooling at fractures.
- How do you care for Onyx?
- Durable; avoid harsh solvents that can lift dye on treated pieces.
See it in person
Photographs only go so far
Both rooms are open six days a week, and there is always someone happy to pull pieces out of the case so you can compare them under the light.