Chalcedony & jasper

What holds the family together

Chalcedony is silicon dioxide, the same as quartz, but crystallised in bundles of submicroscopic fibres rather than in visible crystals. That single structural difference produces everything that distinguishes the family: it is opaque or translucent rather than clear, it takes a very fine polish, it carves without splitting, and it can build up in layers to produce banding.

Agate is chalcedony with visible banding, usually formed in a cavity where silica-rich fluid was deposited in successive pulses. Onyx is agate with straight parallel bands, which is what cameo carvers wanted, since it lets you cut a white figure from an upper layer against a dark background. Jasper is chalcedony with so much included material, usually iron oxide, that it becomes fully opaque. Carnelian and sard are chalcedony coloured orange to brown by iron.

The whole family sits at hardness 7 with no cleavage, which is why so many small carved objects from antiquity survive in this material and so few in anything softer.

How it forms

Most agate forms in gas bubbles in volcanic rock. Lava traps bubbles, groundwater carrying dissolved silica seeps through, and over long periods the silica lines the cavity in successive layers, sometimes with iron and manganese impurities producing bands of different colour.

Jasper often forms as a sedimentary or hydrothermal replacement, where silica-rich fluid gradually replaces existing rock, taking on whatever iron and clay was there. Ocean jasper, orbicular jasper and picture jasper get their patterns from this process.

Chrysoprase is the family's odd case: it is coloured by nickel rather than iron, and it forms in weathered nickel-bearing serpentinite, which is why it comes from a small number of specific deposits, principally in Australia and formerly in Silesia.

The great historic sources were Idar-Oberstein in Germany, where the local agate deposits supported a cutting industry that survives to this day even though the local material ran out, and Ratnapura and Cambay in South Asia.

The stones of this family

• Carnelian (SiO₂ with iron oxide, Mohs 7). The workhorse amulet of the ancient world, from Egyptian tyet knots to the aqiq seal ring of Islamic tradition. Warm, opaque and endlessly carved.
• Agate (banded SiO₂, Mohs 7). Banded chalcedony named for a Sicilian river by Theophrastus. Almost all bright agate slices sold today are dyed, and the dye is not a scandal if it is declared.
• Black onyx (SiO₂, banded, Mohs 7). Properly a banded stone, sold today as a solid black one, and usually dyed grey chalcedony. Cameo carvers built their whole craft on its layers.
• Blue lace agate (SiO₂ with lacy banding, Mohs 7). Found in Namibia in the twentieth century, so it has no ancient lore at all. A useful test case for how quickly a modern meaning gets written.
• Bloodstone (green jasper with hematite, Mohs 7). Green chalcedony flecked red, called heliotrope by the Greeks and tied by medieval Christians to the blood at the Crucifixion. March's older birthstone.
• Red jasper (SiO₂ with iron oxide, Mohs 7). Opaque, dependable and everywhere. Egyptian tyet amulets, Roman signet rings and the fourth-century lapidaries all name it for steadiness rather than brilliance.
• Chrysoprase (SiO₂ with nickel, Mohs 7). Apple-green chalcedony coloured by nickel, prized in Roman jewellery and later by Frederick the Great, who had Silesian mines worked for it.

Trade names and lookalikes

This family carries more dye than any other. The bright blue, green, purple and magenta agate slices in every shop are dyed, using processes documented in Roman sources and industrialised in nineteenth-century Germany. Natural agate colours are browns, greys, creams, whites, soft reds and occasionally natural blue.

Black onyx is almost always dyed grey chalcedony, treated with sugar solution and sulfuric acid to deposit carbon in the pores. This is a Roman technique and the trade regards it as normal.

Carnelian is routinely heated, and much of what is sold as carnelian is heated agate. Blue lace agate, dendritic agate, moss agate and fire agate are natural varieties with genuine identities.

Watch for invented names: a great many jasper trade names describe attractive material from one deposit rather than a distinct mineral, which is harmless as long as it is not sold as rare.

Care that follows from the structure

Structurally this is the easiest family to look after. Hardness 7, no cleavage, chemically inert. Warm water and a soft brush are fine, and it survives daily wear better than almost anything else in the catalogue.

The one real caution concerns dye. Dyed material can fade in strong sunlight and can bleed if soaked in hot water or solvents, so if you own a bright blue agate slice, keep it out of direct sun and clean it dry.

Porous jasper and chalcedony can absorb oils over time, which will darken a piece unevenly. This is slow and largely cosmetic.

Chalcedony is safe for indirect gem waters and, in undyed natural form, for direct contact too, being simply silica. That does not extend to dyed material, where you do not know what the dye was.