Agate

Quartz (chalcedony) · also called Banded chalcedony, Fortification agate

Agate is the banded variety of chalcedony, the microcrystalline form of quartz, formed by successive layers of silica deposited inside a cavity in volcanic rock. Everything that distinguishes one agate from another is structure and trace impurity rather than chemistry. It is also the most heavily dyed material in the gem trade.

What is agate?

Agate is the banded variety of chalcedony, the microcrystalline form of quartz. Its bands are successive layers of silica deposited from solution inside a cavity, most often a gas bubble in a cooling lava. Chemically it is nothing but silicon dioxide; everything that distinguishes one agate from another is structure and trace impurity.

The material is an intergrowth of fibrous quartz with a variable proportion of moganite, a closely related silica polymorph, and it is this fibrous, interlocking texture that gives chalcedony its toughness. Under the microscope the fibres run perpendicular to the banding, recording the direction in which each layer grew inward from the cavity wall. Where the layers follow the irregular outline of the cavity the result is fortification agate, named for the resemblance to the angular plan of a bastioned fort. Where they lie flat and parallel, the material is banded onyx or, with a brown layer, sardonyx.

What gives agate its bands and colours?

The banding is a growth record, not a colour effect. Each layer represents a change in the silica-bearing fluid — its concentration, its temperature, its trace chemistry — and the layers differ in fibre size, water content and porosity as well as in impurity. Colour is supplied mainly by iron compounds, with manganese, nickel and chromium contributing in particular deposits.

Iron oxides and hydroxides account for most of what is seen naturally: goethite and limonite for yellow and brown, haematite for red, with the red usually a dehydration product of the yellow rather than an independent addition. Greens generally come from included chlorite or celadonite, and the blue-greys of much untreated agate are a structural effect — fine-scale light scattering rather than a colouring ion at all.

The critical point for a buyer is that the layers differ in porosity as well as in colour. Some bands will take up a dye solution readily and others will not, which is what makes agate the most heavily dyed material in the gem trade and also what makes the dyeing detectable.

How is agate identified?

The numbers are those of quartz, because agate is quartz. Refractive index reads about 1.53 as a spot reading, specific gravity falls between 2.58 and 2.64, and hardness is 6.5 to 7. The slightly low specific gravity relative to single-crystal quartz at 2.65 reflects the porosity and the water content of the fibrous structure.

Because it is a polycrystalline aggregate, agate gives an aggregate reaction in the polariscope — it stays light through a full rotation between crossed filters rather than blinking dark four times. That single observation separates it from most single-crystal look-alikes.

The banding itself is the practical identifier. Natural agate banding is continuous, curved or angular in a way that follows the cavity wall, and it passes through the stone rather than sitting on the surface. Glass imitations show banding that is swirled and stretched, with the elongated gas bubbles that come from working a viscous melt; plastic imitations are warm to the touch and read far below quartz on any hardness test.

The distinguishing feature: what the bands do to light

Agate has one optical behaviour that belongs entirely to it. Where the banding is fine and regular enough — on the order of the wavelength of light — a thin slice cut across the bands acts as a diffraction grating and returns full spectral colours in transmitted light. This is iris agate, and it is a genuine structural phenomenon rather than a treatment or an inclusion effect.

A second effect belongs to fire agate, found chiefly in Mexico and Arizona. Here the silica has grown over botryoidal layers of iron hydroxide, and thin-film interference between those layers produces a metallic play of red, green and gold beneath the surface. Cutting it is unusually difficult, because the colour lies in a specific layer and the cutter must grind down to it and stop.

Neither effect can be convincingly manufactured, which makes both useful. Fine iris agate and true fire agate are among the very few chalcedony products where the material itself, rather than the polish and the pattern, carries the value.

Is agate treated?

Constantly, and it has been for two centuries. Dyeing is the norm rather than the exception in commercial agate, and the trade generally treats it as understood rather than as something to declare, which is exactly why a buyer has to ask.

The classic methods were developed at Idar-Oberstein in Germany in the nineteenth century and are still in use. Soaking in a sugar solution followed by concentrated sulphuric acid carbonises the sugar within the porous layers and produces the dense black sold as black onyx; almost all black onyx in commerce is treated grey agate. Iron salts give red and brown, potassium ferrocyanide compounds give blue, chromium compounds give green. Heating alone converts yellow goethite to red haematite and is how most carnelian reaches its colour.

The tell is in the porosity. Dye concentrates where the stone would take it up, so colour follows the banding as a series of sharply bounded, unnaturally even bands, or pools in fractures and along the surface. Under magnification, dyed blue and green agate often shows colour concentrated in the interstices between fibre bundles. Improbable colours — bright blue, purple, magenta — should simply be assumed to be dyed.

Impregnation with wax or polymer to improve polish is also common on porous material, and dyed agate is sometimes further stabilised this way.

Where does agate come from?

Southern Brazil and Uruguay supply the bulk of the world's agate. The basalt flows of the Paraná basin, worked around Salto do Jacuí in Rio Grande do Sul and across the border at Artigas in Uruguay, hold enormous numbers of amygdaloidal cavities and have fed the cutting trade since the 1830s. Uruguayan material is typically darker and more strongly banded; Brazilian material is often grey and is the principal feedstock for dyeing.

India is the other great historic source, from the Deccan traps, with the cutting and drilling industry concentrated at Khambhat in Gujarat — a bead-making centre with a continuous history running back several thousand years.

Mexico produces the highly regarded patterned agates of Chihuahua, including Laguna agate from Ojo Laguna and the crazy lace material of the same region. Botswana yields the fine grey and pink banded agate that carries the country's name, from the Bobonong area. Namibia is the source of blue lace agate. Scotland's Ayrshire and Angus coasts produced the small, richly coloured nodules known historically as Scotch pebbles, and Germany's Idar-Oberstein district — the cutting capital — exhausted its own local deposits long ago and has imported rough ever since.

Where the name comes from

The name is recorded by Theophrastus in the fourth century BC and repeated by Pliny, both deriving it from the Achates, a river in Sicily now generally identified with the Dirillo. Agate is therefore one of the oldest continuously used gem names in any European language.

The material's history is older still. Banded agate beads appear in Mesopotamian and Indus Valley burials, and the etched carnelian beads of the third millennium BC — patterned with an alkali paste before firing — are among the earliest deliberate gemstone treatments known. Roman cameo and intaglio cutters exploited the layering directly, carving through a pale band into a dark one to raise a white figure against a dark ground, a technique that depends entirely on agate's structure and has no equivalent in a single-coloured stone.

What sizes are available?

Effectively unlimited. Agate nodules from the Paraná basin regularly reach tens of kilograms, and geodes over a metre across are cut, split and sold as furniture. Nothing about size constrains what can be made from agate, which is why it dominates the market for bookends, slices, coasters, spheres and large decorative work.

Because size is free, it contributes almost nothing to price. Value in agate attaches to pattern, to colour, and to the quality of the polish. A palm-sized Laguna agate with an exceptional fortification pattern will outsell a fifty-kilogram grey geode many times over.

How much is agate worth?

Agate is an inexpensive material with a small number of expensive exceptions, and the whole of the price question lies in identifying the exceptions.

Pattern comes first. Named patterned agates with a following — Laguna, crazy lace, plume, moss and dendritic material with well-formed inclusions — command real prices per piece, driven by how sharp, complete and well-centred the pattern is and by how well the cutter has oriented the slice. Natural colour comes second: an untreated agate with saturated natural red or blue banding is worth substantially more than a dyed one, and blue lace agate of good even colour is priced well above generic banded material.

Then come the optical rarities, iris and fire agate, where the effect itself is the product. Everything else — plain grey, dyed, generic banded — is sold by weight and is genuinely cheap. It is worth being clear-eyed about that, because the same word covers a two-pound bookend and a collector's slab.

Is agate durable enough to wear?

Very. At 6.5 to 7 agate sits at the top of the range where airborne dust, which contains quartz, will not readily abrade it, and its interlocking fibrous structure makes it markedly tougher than single-crystal quartz. Agate has no cleavage. It has been used industrially for mortars and pestles, for burnishing tools and for the knife-edge bearings of precision balances precisely because it resists both wear and impact.

The cautions concern treatment rather than the stone. Dyed agate can fade with prolonged strong sunlight, and dye can leach if the piece is exposed to solvents or to ultrasonic cleaning. Wax-impregnated material will lose its polish if heated. Untreated agate needs no special care beyond warm soapy water; treated agate should be kept away from ultrasonics, steam and household chemicals.

How to buy agate without being caught out

Start from the assumption that the colour is treated and let the seller talk you out of it. Bright blue, purple, magenta and dense uniform black are not natural agate colours in commercial quantity, and a seller who cannot say whether a stone is dyed is telling you something either way.

Look at where the colour sits. Natural colour follows the growth banding with soft, variable boundaries and varies in intensity along a single band; dye produces flat, uniform, hard-edged bands, and pools visibly in fractures and around the drill hole of a bead. Turning a bead so the hole is visible under a loupe is the single most useful test in the material.

Judge the polish and the orientation of the cut, because in patterned agate these are most of the value. And when buying a named locality material — Laguna, Botswana, blue lace — remember that these are trade names attached to appearance as often as to a mine, so buy the stone in front of you rather than the label. Browse our gems or our specimens, or tell us what you are looking for.

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