Agate

What it actually is

Chalcedony, silicon dioxide in submicroscopic fibres, deposited in successive layers. Hardness 7, no cleavage, which makes it one of the most durable materials in the catalogue.

Most agate forms in cavities in volcanic rock. Lava traps gas bubbles, silica-rich groundwater seeps through over long periods, and successive depositions line the cavity. Trace iron and manganese produce colour differences between bands.

The named varieties describe patterns rather than different minerals. Fortification agate has angular concentric bands; moss agate and dendritic agate contain mineral inclusions that look like vegetation but are not; blue lace agate is finely banded pale blue; fire agate shows iridescence from thin iron oxide layers.

Onyx is agate whose bands run straight and parallel, which is what cameo carvers require.

Where it comes from

Brazil and Uruguay dominate world supply, producing the large geodes and slices that fill every shop. India, particularly the Deccan traps, has been a source for millennia. Botswana produces a distinctively fine-banded grey and pink material.

Idar-Oberstein in Germany is the historic cutting centre. Local agate deposits supported an industry from the medieval period, and when the local material ran out in the nineteenth century the cutters simply imported Brazilian agate and kept working. The town still cuts stone today, and the industrial dyeing techniques that colour most of the world's agate were developed there.

Sicily's Achates river, now the Dirillo, gave the stone its name via Theophrastus around 315 BCE.

What the traditions claimed

Agate's role in the ancient world was practical before it was symbolic. Mesopotamian and Indus cylinder seals in agate and chalcedony were rolled across wet clay to sign documents, and a seal was a legal identity. Roman signet rings continued the practice, and cameo carving exploited the banding directly, cutting a white figure from an upper layer against a darker one.

The lapidary tradition gives agate protective claims. Pliny records a belief that agate makes athletes invincible, which he reports with visible scepticism, and later sources give it protection against storms and against snakebite, and particular value for farmers.

Medieval European lapidaries following Marbode credit it with making the wearer agreeable and persuasive.

Modern practice reads agate as grounding and stabilising, which is a reasonable summary of a stone whose whole character is slow accumulation.

Pliny is also the earliest witness to the other half of agate's story. Book 37 of the Natural History describes both the heating of agate and the dyeing of chalcedony, which means the treatments filling shop shelves today were already in use in the first century, and so was the worry about whether a buyer had been told.

How it is worked with

The best stone in the catalogue for anything you intend to carry roughly. Hardness 7, no cleavage, chemically inert, and cheap enough to replace.

The layering reading is the useful one: agate is a record of successive deposits, and it suits questions about things built slowly, about patience, and about what has accumulated without your noticing.

Slices and geode halves are the common decorative form; tumbled pebbles and palm stones the practical one.

If you buy one piece of agate, consider buying natural rather than dyed. The browns, greys, creams and soft reds are the honest colours, and a naturally banded piece is a more interesting object than a fluorescent one.

Buying it honestly

This is the most dyed material on the market. The bright blue, green, purple, magenta and turquoise slices in every shop are dyed, using processes documented in Roman sources and industrialised at Idar-Oberstein in the nineteenth century.

Dye is not fraud when declared, and much dyed agate is beautiful. It becomes a problem when a dyed piece is sold as a rare natural colour at a premium.

Natural agate colours are browns, greys, creams, whites and soft reds, with occasional genuinely natural blues and greens.

Under a loupe, dye pools along band edges and in cracks. Acetone on a cotton bud will lift many dyes.

Dyed material can also fade in strong sunlight and bleed if soaked in hot water or solvents, so keep bright slices out of the window and clean them dry.