Agate
Chalcedony (quartz)JewelryWiki Editorial·2 sources·Reviewed 2026-08-04
Chalcedony with curved or concentric banding, formed as layers lined the inside of a cavity in volcanic rock. The banding is the definition: straight parallel bands make it onyx, curved ones make it agate. Much of the vividly coloured agate on sale is dyed, which the banding makes easy to spot once you know to look — dye concentrates in the more porous layers.
Properties
| Property | Value | Authority |
|---|---|---|
| Mohs hardness | 7 [1] | — |
| Specific gravity | 2.55–2.91 [1] | — |
| Chemical formula | SiO2 Read from the species entry, Chalcedony. A variety shares its species’ formula. [2] | — |
| Crystal system | Hexagonal (trigonal), microcrystalline [1] | — |
| Common treatments | Frequently dyed. IGS lists dyeing among chalcedony’s typical treatments, and GIA illustrates a single chalcedony slice cut into sections each dyed a different colour. Improbable colours — electric blue, magenta — are dyed. [1] | — |
| Care cautions | Avoid acetone and alcohol, which can lift dye, and prolonged sunlight, which can fade it. [1] | — |
Sources
- International Gem Society — gemstone articles — International Gem Society (IGS, gemstone information articles (gemsociety.org))
- Handbook of Mineralogy — Mineral Data Publishing / Mineralogical Society of America (Handbook of Mineralogy, Mineral Data Publishing)