Gem library
Agate
Quartz with a thousand ribbons
Agate is banded chalcedony: silica deposited in successive concentric layers inside a rock cavity. Each band records a change in chemistry or temperature — a cut agate is literally a geological archive.
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.58 – 2.64
- Crystal system
- Trigonal
- Refractive index
- 1.530 – 1.540
- Cleavage
- None
Mineralogical identity
- Family
- Microcrystalline quartz — chalcedony
- Species
- Chalcedony
- Variety
- Agate (banded chalcedony)
- Chemical formula
- SiO₂
- Crystal system
- Trigonal (microcrystalline aggregate)
- Classification
- Semi-precious stone
- Rarity
- Very common
- Indicative price
- €0.20 – 5/ct
Etymology
From the Achates, a Sicilian river (today the Dirillo) where Theophrastus placed its discovery in the fourth century BC.
Physical & optical properties
- Hardness (Mohs)
- 6.5 – 7
- Density
- 2.58 – 2.64
- Refractive index
- 1.530 – 1.540
- Birefringence
- up to 0.004
- Cleavage
- None
- Fracture
- Conchoidal to splintery
- Lustre
- Vitreous to waxy
- Diaphaneity
- Translucent to opaque
- UV fluorescence
- Weak
- Pleochroism
- None
- Colours
- multicoloured, brown, blue, grey, white, red
Formation, geology & history
How the bands form
A gas bubble trapped in a lava flow creates a cavity. Silica-bearing water enters and deposits silica layer by layer, each stratum taking the tint of the impurities present at that moment: iron for red and brown, manganese for pink, chlorite for green. The banding is therefore concentric and follows the shape of the cavity, sometimes interrupted by a horizontal gravity fill (water-level agate). When the layers are parallel and straight it is called banded onyx; when they form circles and eyes, eye agate.
The great chalcedony family
Agate is only one form of chalcedony, that fibrous microcrystalline quartz whose structure is too fine to resolve with an optical microscope. Its cousins carry familiar names: carnelian (orange-red), chrysoprase (apple green, nickel), sard (brown), heliotrope or bloodstone (green flecked with red), onyx (black), and jasper itself (opaque and rich in impurities). All share the same formula, hardness and absence of cleavage, which makes them easy to work and remarkably durable.
Dyes and treatments
Agate is porous on a microscopic scale, so it takes dye — and has done since Roman antiquity. Idar-Oberstein in Germany industrialised the technique in the nineteenth century: iron nitrate baths for red, honey followed by sulphuric acid for the black of onyx, nickel salts for green. The electric blue, magenta and turquoise agates seen in shops are always dyed: no natural agate reaches those saturations. The treatment is stable and ancient, but it must be declared.
How to recognise it
- Concentric bandsBanding that follows the outline of a cavity, with layers of irregular thickness, is the agate signature. Over-regular bands suggest an assembly or glass.
- Hardness 6.5-7It scratches glass. A banded stone that a knife scratches is more likely banded calcite (so-called Mexican onyx) or aragonite.
- Waxy lustre on fractureAgate's fracture is slightly splintery and waxy, never as vitreous as that of coarse-crystalline quartz.
Imitations, treatments & confusions
- Dyed agate presented as naturalElectric blue, fuchsia pink, bright green: those tones do not exist in nature. The dye is often more intense in the porous layers, creating a very sharp artificial contrast.
- Banded glassFake glass agate slices show over-parallel bands, bubbles, and warm quickly in the hand.
- Resin-impregnated slicesThe very thin translucent slices sold as decoration are sometimes consolidated with resin; a hot-needle test in a hidden area releases a plastic smell.
Deposits & origins
Brazil and Uruguay (giant banded agates), India, Botswana (highly prized grey-pink agate), Madagascar, Mexico (fire agate, lace agate), Germany (Idar-Oberstein, the historic capital of gem cutting), Scotland
Care & precautions
- No constraints: water, soap, sun and salt do not affect it.
- If the stone is dyed, avoid hot water and solvents, which leach the colourant.
- The polish of large slices is refreshed with fine polishing paste on felt.
- Ultrasonic-safe, except on broken and reglued pieces.
Traditions & crystal healing
- Chakras
- Varies with colour
- Zodiac signs
- Gemini · Taurus
- Element
- Earth
A stone of balance in tradition: it is said to steady emotions without dulling them, and its banding makes it a symbol of patience and slow construction. Eye agate was worn as an amulet against the evil eye throughout the Mediterranean basin — a use attested since Sumer.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is my agate dyed?
Look at the saturation: electric blues, bright pinks and apple greens are always artificial. Natural agates stay in greys, whites, browns, ochres and earthy reds.
Agate or jasper?
A matter of translucency: agate transmits light at thin edges and shows banding; jasper is fully opaque and its pattern is blurred and mottled rather than banded.
Why are agates bubble-shaped?
Because they fill former gas bubbles frozen in lava. The geode's shape follows that of the original bubble, often spherical or almond-shaped.