Agate

How agate forms, and why it is banded

Agate grows inside a hole. When lava cools, trapped gas leaves cavities, and silica-bearing water seeps in over thousands of years and deposits layer on layer against the cavity wall. Each layer records a slightly different chemistry, so each has a slightly different colour and transparency. That is the banding.

Because the layers are concentric, the pattern only reveals itself when the nodule is cut across. This is the single most important practical fact about agate: it is a slicing stone, not a faceting stone. A faceted, transparent, evenly coloured "agate" is almost certainly something else — or dyed chalcedony with the banding polished away.

The last thing to fill is the middle. Often the water runs out before the cavity closes, and the centre is left as a cavity lined with quartz crystals. That is a geode. Agate and geode are the same event, stopped at different points.

Where agate comes from

The volume of the world's agate comes from the basalt flows of southern Brazil and northern Uruguay, where the Paraná traps are riddled with gas cavities. Rio Grande do Sul in Brazil and Artigas in Uruguay are the two names that matter commercially, and between them they supply most of what is cut, dyed and sold worldwide.

Idar-Oberstein in Germany is not a source any more but is still a centre: it cut local agate from the fifteenth century, ran out, and turned itself into the processing and trading hub for Brazilian material instead. Much of the world's agate cutting vocabulary is German for that reason.

Botswana produces a fine grey-pink banded agate sold under its own name. Madagascar produces ocean jasper and a range of banded material. India, Mexico, Morocco and the United States all produce agate with local trade names — Laguna, Crazy Lace, Fairburn, Dryhead — most of which describe a pattern rather than a place.

Is agate dyed?

Usually, yes. Agate is the most commonly dyed gem material in the world, and it has been dyed industrially since the nineteenth century in Idar-Oberstein. It takes dye well because the layers are slightly porous and absorb at different rates, which is exactly what makes a dyed stone look convincing — the bands still differ from one another.

The tell is the colour itself. Natural agate runs to muted browns, greys, creams, honey and soft blue-white. Bright magenta, electric blue, lime green and purple agate are dyed, without exception. Black onyx — banded agate dyed black with sugar and sulphuric acid — is a centuries-old accepted treatment, but it is still a treatment and should be stated.

Dyeing is not fraud when it is disclosed, and much of the trade regards it as normal. It becomes fraud at the point where a dyed stone is sold as a natural colour. We state it.

What agate is used for

Agate is the clearest example of why one stone is not one market. The same species goes into six unrelated trades, and each one wants something different from it.

Jewellery wants cabochons, beads and layered material for intaglio and cameo — a carver cuts through a white layer into a brown one to get a two-tone relief. Architecture and interiors want large slabs, book-matched and backlit, where the translucency of the bands becomes the whole point of the wall.

Laboratories want agate mortars and pestles, because agate is hard, dense and chemically inert, so it grinds a sample without contaminating it. Gilders and bookbinders want agate burnishers — a shaped agate tool is still the only thing that brings gold leaf to a mirror polish, and nothing synthetic has replaced it.

Prayer-bead makers in the Gulf and Turkey grade agate on the evenness and match of the banding across a strand. Collectors want sliced nodules, geodes and cathedral specimens, where the value is the pattern and the completeness, not the carat.

What agate costs

Agate is not priced per carat in most of its markets, which is itself a useful thing to know. Cut cabochons and beads trade by the piece or the strand; slabs trade by the square metre; specimens trade as objects.

As a rough guide: commercial cabochons and bead strands sit between $0.50 and $20 per carat of finished stone. Fine natural-colour banded material with an unusual pattern — a well-centred eye, a clean fortification, a Botswana grey-pink — runs higher. Architectural slabs of agate, assembled and resin-backed for backlighting, sit between $300 and $1,200 per square metre depending on size, thickness and how well the slab is matched.

Large geodes and cathedral specimens are priced as individual objects and can run into thousands. There is no carat price for them and anyone quoting one is guessing.

How to tell agate from what imitates it

Glass is the common imitation in beads and cheap carvings. Look for gas bubbles and swirl under magnification, and for bands that are too regular — natural banding varies in thickness along its length, printed or moulded banding does not.

Dyed agate is not an imitation but a treated stone, and the giveaway is colour concentration in cracks and at the boundary between layers, where the dye has pooled. Under magnification the colour often sits in the porous bands and skips the dense ones entirely.

Jasper is the same mineral family but opaque and unbanded, coloured by included clay and iron oxide. If light passes through the thin edge, it is chalcedony; if nothing passes, it is closer to jasper. The boundary is a gradient, not a line, and the trade has never agreed on where it falls.

Caring for agate

At 6.5 to 7 on the Mohs scale with no cleavage, agate is one of the more durable materials in this catalogue and takes daily wear well. It will scratch against quartz dust, which is in ordinary household dirt, so it dulls over years rather than chipping.

Warm water, mild soap and a soft brush. Avoid ultrasonic cleaners on dyed material and on any piece with a visible fracture, and keep it out of prolonged strong sunlight — some dyes fade, and a faded dyed stone cannot be brought back.

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