Banded chalcedony — quartz in microscopic fibres. Hardness 7, waxy rather than glassy, and dyed on an industrial scale.
Brazil, Uruguay, India, Mexico, Germany (Idar-Oberstein), Scotland5 field tests
Agate is quartz that grew as microscopic fibres rather than visible crystals, deposited in bands inside a cavity. The banding is a growth record, laid down layer by layer as silica-rich water passed through, which is why the bands are concentric and follow the shape of the void they filled.
It behaves like quartz because it is quartz: hardness 7, no cleavage, conchoidal fracture, inert to acid. What differs is the lustre, which is waxy or resinous rather than glassy, because the surface is a mat of fibre ends rather than a single crystal face.
The thing to know before buying is that agate is dyed on an industrial scale, and has been since the nineteenth century when the trade at Idar-Oberstein in Germany perfected the process. Bright blue, purple, green, magenta and neon-pink agate slices do not occur naturally. They are dyed, and the trade has been open about it for two hundred years — which does not stop them being sold as natural.
Agate's identification is straightforward because it is quartz with a distinctive structure. The bands, the waxy lustre and hardness 7 together are close to conclusive; the real work is in reading whether the colour arrived with the stone or afterwards.
The natural agate palette is narrow: white, grey, cream, brown, rust-red, black, and pale blue-grey. Carnelian's orange-red and the browns come from iron. Everything vivid is dye.
| Property | Agate | Note |
|---|---|---|
| Mohs hardness | 6.5 – 7 | Scratches glass; a knife will not mark it |
| Specific gravity | 2.58 – 2.64 | Slightly below crystalline quartz |
| Lustre | Waxy to resinous | Not glassy — the fibre structure dulls it |
| Fracture | Conchoidal | As all quartz |
| Banding | Concentric, following the cavity wall | Parallel banding is usually onyx |
| Reaction to acid | None | Distinguishes it from banded calcite and marble |
| Natural colours | White, grey, cream, brown, rust, black, pale blue-grey | Vivid blue, purple, green, pink: dyed |
| Translucency | Usually translucent at the edges of bands | Fully opaque banded stone may be jasper |
Banded calcite and travertine are sold as agate reasonably often and are separated instantly by a knife and a drop of vinegar — soft, and it fizzes.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Natural agate runs white, grey, cream, brown, rust-red, black and a soft blue-grey. It does not come in electric blue, violet, emerald, magenta or neon pink. Those slices — the ones sold as coasters and pendants — are dyed, generally after the stone has been soaked in a metal salt solution and heated. Bright colour is not a sign of a fake stone; it is a sign of a real stone that has been coloured.
Dye enters through the more porous layers, so on a dyed slice the colour is strongest in specific bands and weak or absent in the dense ones — producing a striped effect where some bands are vivid and others nearly white. Natural agate colour varies by band too, but gradually and in the muted natural palette. Under a loupe, dyed material also shows colour concentrated along fractures.
Agate scratches glass and resists a steel knife. Banded calcite, travertine and marble are all much softer and all fizz in vinegar, and they are sold as agate often enough to be worth ten seconds. A banded stone a knife cuts is a carbonate, not agate.
Because agate is a mat of microscopic fibres, its polished surface has a soft waxy or resinous sheen rather than the bright vitreous flash of a crystal face. Glass imitations look glassy, which in this one case is the wrong answer.
Agate filled a void, so its bands are concentric and echo the shape of the wall they grew against, often with a central cavity, a quartz-lined pocket, or a straight-banded 'water level' floor at the bottom. Glass imitations have bands that swirl arbitrarily; poured resin has bands that show flow lines rather than growth.
Agate itself is cheap and abundant, so nobody manufactures it for gain. What happens is colouring, and it happens on an enormous scale — the majority of agate slices in a gift shop have been dyed, and the practice is old and well documented rather than clandestine.
The genuinely misleading products are elsewhere: banded calcite sold as agate, resin imitations of large geode slices, and the widespread use of 'onyx' for dyed black chalcedony. Almost all solid black onyx in jewellery is dyed grey chalcedony, and that has been the trade norm for so long that it is essentially the accepted meaning of the word.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
Translucent chalcedony with green mineral inclusions that look like foliage. Not banded, despite the name.
Read within the checks aboveSpecimen figures rather than gem figures, for material in honest condition. Most finds are worth very little, and saying so plainly is more useful than encouraging you.
| Form | What it typically fetches | Note |
|---|---|---|
| Tumbled agate | $1 – 5 | Abundant |
| Dyed slice or coaster | $5 – 30 | Priced by size; the colour was added |
| Natural banded agate, hand-sized polished | $10 – 60 | Band quality and pattern |
| Fine collector agate, named locality | $100 – 1,000+ | Pattern, locality, condition |
| Thunder egg or geode, split pair | $15 – 150 | Interior pattern |
Agate is a decorative material and priced on pattern. Collector premiums attach to named localities and to unusual patterns such as fine Brazilian lace or Scottish Blue Hone.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.