Is Black Onyx Dyed? Material and Treatment

Chalcedony is fibrous microcrystalline silica, mainly quartz with variable amounts of moganite. Its fibres are far too small to see unaided. In a banded piece, the lines you can follow across the cut surface come from differences in texture, porosity and inclusions between neighbouring layers.

The word onyx also appears on carved carbonate objects. Those belong to another material family. For jewellery, read the material name and colour treatment together; a dark colour and a bright polish can occur in several different stones.

Three uses of the word onyx

Onyx is a variety name within the chalcedony family. It describes the arrangement of the bands. A cut through several layers can show alternating light and dark lines; a cut within one layer can show a much more even field. The orientation of the cut changes what appears on the surface.

Chalcedony is the broader material name. Agate is banded chalcedony, and onyx names material with nearly straight, parallel layers. These are related names within the same material family. An agate can have parallel bands, so curved lines and straight lines do not divide them into two separate mineral species.

Sardonyx combines reddish-brown sard-coloured layers with contrasting lighter layers. Nicolo is a historical name for a thin bluish layer over a darker base. Both names describe layered chalcedony. The pale lines and dark fields made layered material useful for carvings, with a figure cut in one colour above a background in another.

Onyx marble belongs with decorative carbonate stone. Its composition and hardness differ from chalcedony. The name therefore needs to be read in its setting: a lamp base, a carved bowl and a strand may use the same word while naming different materials.

In 1992, Peter J. Heaney and Jeffrey E. Post examined more than 150 fine-grained quartz specimens from around the world. They found variable amounts of moganite within both fibrous and nonfibrous materials. Their work helps explain why a description of chalcedony includes its fine texture as well as the silica phases it contains.

What "dyed" means for black onyx

Sugar and sulfuric acid carbonisation is one documented method for blackening chalcedony. Sugar enters the porous material, and the acid carbonises it. The resulting carbon remains in the pores. GIA laboratory notes have identified carbonised sugar in black chalcedony using Raman and infrared analysis.

Natural describes material formed in nature. A natural gemstone can later receive a colour treatment. Its treatment entry states what was done to the particular piece.

Chalcedony can also take other dyes, and a surface coating is another documented treatment. These methods put colour in different places. A coating lies on the surface; a dye or carbonisation treatment works in porous parts of the material. The treatment entry tells you which description applies to a particular piece.

A dark layer can absorb treatment differently from the layer beside it because the porosity varies. That difference has long been useful in working with layered chalcedony. A colour treatment can deepen the contrast while the underlying layers remain part of the stone.

In its 1986 Gem News, GIA described virtually all so-called black onyx as chemically treated chalcedony. That is a description of commercial material. A statement about an individual strand belongs with that strand’s own treatment record.

CIBJO’s 2026 Gemstone Blue Book lists black onyx as “Dyed (always)” and straight-layered black-and-white onyx as “Dyed (commonly)”. The frequency column reflects the judgement of gemmological laboratories. These entries describe material encountered in trade. An individual strand has its own treatment record.

A 1988 GIA laboratory note described a strand containing both dyed black chalcedony and brown banded agate with a black surface coating. The beads shared a black appearance, produced by two different treatments. The laboratory recorded the material beneath the coating and the treatment on it as distinct parts of the description.

Looking closely at a strand

A polished round bead shows the part of the material chosen by its cut. Layers may cross the face, run near a drill hole or sit beyond the visible surface. Turn the strand slowly in daylight and look at more than one bead. A pale line may be easier to follow from the side than from above.

1. Start with the material entry. Chalcedony names the material family. Onyx describes its nearly straight, parallel banding.
2. Read treatment separately. Natural material can undergo colour treatment. Keep the material description and treatment description together.
3. Turn the beads under one light. Follow any visible bands across the surface and notice how their position changes from bead to bead.
4. Look at the finish. View the polish and the edges of drill holes closely. Use the Black Bead ID for a wider comparison of black materials.

Appearance alone cannot establish the identity and treatment of an opaque black bead. Gemmological identification combines properties such as microscopic structure, refractive index and specific gravity, with further analysis where needed.

Colour and layers

The banding in chalcedony forms as silica grows. Neighbouring bands may differ in texture, porosity, inclusions and silica phases. One dark band can have a different appearance from the next without representing a different mineral species.

The cut decides how much of that pattern reaches a bead’s surface. A layered rough piece can produce a bead with a broad dark area, a fine pale line or several visible stripes. Those are different views into the same layered material.

Yifeng Wang and Enrique Merino proposed a self-organising model of agate banding in 1990. In that model, changes in the growing material and oscillating precipitation produce the bands. A band records a change during growth. Counting the lines gives you the visible pattern; a geological age needs other evidence.

Colour, pattern and treatment are useful to read together. For comparisons with volcanic glass, Obsidian vs Onyx sets the two materials beside each other.

The name onyx in historical records

The name onyx comes through Latin from the Greek ὄνυξ, meaning “claw” or “fingernail.” An Athenian temple inventory dated 422/1 BCE lists an onyx with a gold ring. The word has travelled through centuries of gemstone names.

Pliny the Elder's Natural History describes onyx through white bands and contrasting colours. Ancient stone names followed appearance; the modern trade name black onyx also covers colour-treated chalcedony. The Metropolitan Museum of Art catalogues a Roman sardonyx cameo dated to the first century BCE through the third century CE, an example of layered stone used in carving.

Living with it

For everyday care, wipe the strand with a soft cloth and keep it away from cleaning chemicals. Store pieces separately and follow the jewellery care guide for water and cleaning advice.

How BE. records its stones

BE. uses Crystal 4T™ to describe material quality. The Stone Origin Record brings the stone identity, treatment and origin together with the four readings. Read those details alongside the piece you are choosing from the BE. collection.

References

• Heaney and Post: silica phases in microcrystalline quartz
• Wang and Merino: self-organizational origin of agates
• GIA: identification of black opaque gem materials
• GIA: Fall 2003 laboratory notes
• GIA: Winter 1986 Gem News
• Götze, Möckel and Pan: mineralogy and genesis of agate
• Mindat: onyx
• CIBJO: 2026 Gemstone Blue Book, treatment frequency and disclosure