Agate is banded chalcedony — microcrystalline quartz — and it is porous enough to absorb dye and stable enough to hold it. Since the nineteenth century, when the Idar-Oberstein cutters industrialised the process, dyeing has been the normal way of producing agate in blue, bright green, hot pink, purple, red and black. Natural agate is mostly grey, white, pale brown and muted red. A vivid agate slice, sphere or bead should be assumed dyed until shown otherwise.
How agate is dyed
The stone is soaked in a chemical solution that penetrates the porous bands, then treated to fix the colour: iron salts for red and brown, chromium for green, cobalt or ferrocyanide for blue, sugar and acid (carbonising) for black, aniline dyes for pink and purple. Different bands absorb differently — the more porous layers take more dye — which is why dyed agate often shows exaggerated contrast between bands. The colour is permanent and does not fade in normal use.
Detecting dye
Colour. Bright blue, vivid green, hot pink, purple and uniform jet black do not occur naturally in agate. Turquoise-blue agate does not exist undyed.
Band contrast. Unnaturally sharp contrast between deeply coloured and white bands, with the colour concentrated in the porous layers, is characteristic of dye.
Dye concentration. Under magnification, dye collects in surface fractures, pits and along the drill hole, and may show as dark lines in cracks. Natural colour does not follow cracks.
Spectroscope. Some dyes show characteristic absorption; carbonised black agate is opaque and read differently.
Solvent swab. A cotton swab with acetone applied to an inconspicuous spot lifts some aniline dyes. The laboratory prefers non-destructive methods and uses this sparingly.
UV. Some dyes fluoresce; some natural agate does too, so this is supporting evidence.
The "natural" grey-and-white agate is often also treated
Natural agate that is sold as untreated has frequently been heated to improve the red and brown tones (iron-bearing bands turn redder on heating). This is a minor, traditional treatment, but it is a treatment.
Named varieties and what they usually are
- Blue lace agate — a naturally pale blue-and-white banded agate; genuine, but imitated by dyed material and by dyed white agate in "blue lace" patterns that are too vivid.
- Moss agate, dendritic agate — natural inclusions of green or black minerals; not usually dyed.
- Fire agate — natural iridescent layers; not dyed.
- Black onyx — almost always dyed (carbonised) agate; natural black chalcedony is uncommon.
- Green onyx — dyed.
- "Crackle" agate, "dragon vein" agate — heated and quenched to induce fractures, then dyed; the fractures hold the dye.
- Carnelian — often heated agate; some is dyed.
- Sardonyx — banded, often dyed.
Why it matters
Dyed agate is inexpensive and there is nothing wrong with buying it. The problem is when it is sold as natural, at a natural price, or as a "rare" colour. A laboratory report identifies the material (chalcedony / agate) and states whether dye is present. For inexpensive beads the exercise may not be worth the fee; for slices, spheres and bookends sold as natural, it is.
Frequently asked questions
Is dyed agate still real agate?
Yes — the stone is agate; the colour is not natural. Both facts should be disclosed.
Does the dye come off?
Fixed dye is permanent in normal wear. Some aniline dyes can be lifted from the surface by solvents, which is one detection method.
Are natural agate colours ever bright?
Red, orange and brown can be quite strong naturally, especially after heating. Blue, green, purple and pink of any vividness are dyed.
How does dyed agate differ from dyed quartz?
Same mechanism, different material. Dyed quartz is crackled clear quartz with dye in the fractures; dyed agate takes dye into its porous bands. Both show dye in cracks under magnification.
Is dyed agate suitable for a mala or bracelet?
For wearing, yes. Whether a tradition accepts dyed material is for the wearer; the laboratory's role is to state that it is dyed.