Chalcedony
Chalcedony is quartz in crystallites 50 to 100 nm across, with no IMA row. Moganite at 5 to 20 wt% and water at 1 to 2 wt% leave 78 to 94% quartz. Hardness 7.
Chalcedony is quartz, SiO2, in fibres of crystallites 50 to 100 nm across, too small to see, with no IMA row of its own. A 2024 Mineralogical Magazine study puts moganite at 5 to 20 wt% and water at 1 to 2 wt%, leaving 78 to 94% alpha-quartz. Hardness 7; agate and onyx are chalcedony.
In short
- Chalcedony is a form of quartz, not a species. A script over the September 2026 IMA-CNMNC Master List finds no row for chalcedony, agate, onyx, carnelian, jasper or chrysoprase. The Handbook quartz sheet describes quartz as fine-grained to microcrystalline (“chalcedony”), massive, with a waxy to dull lustre when massive. There is no Handbook chalcedony sheet.
- The 78-to-94 rule. Monico and co-authors, Mineralogical Magazine 88 (2024) give chalcedony 1 to 2 wt% water and 5 to 20 wt% mogánite, a separate silica mineral with its own IMA row. Subtracting both leaves 78 to 94 wt% alpha-quartz. Chalcedony is mostly quartz, but on these figures not only quartz.
- Fibres, not crystals you can see. The same paper describes chalcedony as alpha-quartz crystallites 50 to 100 nm across in a fibre-like texture, in length-fast and length-slow (quartzine) kinds. The International Gem Society’s definition is the practical one: quartz whose crystals are too small to see without high magnification.
- Agate, onyx, carnelian, chrysoprase and jasper are names within chalcedony on the gem society’s articles. Chrysoprase owes its green to nickel; Monico and co-authors identified the nickel-bearing layered silicate willemseite in it, which has an IMA row, Ni3Si4O10(OH)2.
- Quartz-weight on the scale. The quartz sheet gives 2.65 for crystals and 2.59 to 2.63 when massive. A 20.00 g piece reads 12.45 g in water as crystal quartz and 12.31 to 12.42 g as agate on the gem society’s 2.60 to 2.64, so the scale cannot separate them.
| Name | What it is (source) | Colour cause (source) | Specific gravity | 20.00 g piece in water | IMA row? |
|---|---|---|---|---|---|
| Chalcedony | Microcrystalline or cryptocrystalline quartz (IGS); 78 to 94 wt% alpha-quartz, worked from Monico 2024 | Metallic impurities; unstained material often greyish blue (IGS) | 2.55 to 2.91 (IGS) | 12.16 to 13.13 g | No |
| Agate | Chalcedony with a pattern, or by translucency (IGS); banded chalcedony (Monico 2024) | Iron, manganese, titanium and chromium oxides (IGS) | 2.60 to 2.64 (IGS) | 12.31 to 12.42 g | No |
| Onyx | Chalcedony with straight, parallel bands (IGS) | Black usually dyed (IGS) | 2.651 (IGS) | 12.46 g | No |
| Carnelian | Red, orange-red or brownish chalcedony (IGS) | Iron; assume heated unless told otherwise (IGS) | 2.58 to 2.64 (IGS) | 12.25 to 12.42 g | No |
| Chrysoprase | Translucent green chalcedony (IGS) | Nickel (IGS); willemseite identified (Monico 2024) | 2.620 (IGS opal article) | 12.37 g | No; willemseite yes |
| Jasper | Opaque, solid or patterned cryptocrystalline quartz (IGS) | Mineral impurities (IGS) | 2.55 to 2.91 (IGS) | 12.16 to 13.13 g | No |
| Mogánite | Monoclinic silica of alternating left- and right-handed quartz lamellae; 5 to 20 wt% of chalcedony (Monico 2024) | None stated | No sheet | Not worked | Yes: SiO2·nH2O, Rn, 1999-035, Spain |
| Quartz, massive | Handbook quartz sheet | Inclusions; chemical or particulate (sheet) | 2.59 to 2.63 | 12.28 to 12.40 g | Yes, as quartz |
| Quartz, crystal | Handbook quartz sheet | As above | 2.65 | 12.45 g | Yes: SiO2, A, 1967 s.p. |
Is chalcedony a mineral? The quartz sheet's word for microcrystalline quartz
Not a species of its own. A script over the September 2026 IMA-CNMNC Master List finds no row for chalcedony, and the string chalced appears nowhere in the list. Quartz has a row, SiO2, status A, 1967 s.p. The Handbook of Mineralogy has no chalcedony sheet; chalcedony.pdf returned 404 on 12 October 2026. The quartz sheet itself uses the word, in quotation marks, for the fine-grained to microcrystalline habit, and records a lustre that is waxy to dull when massive.
What other sources call it. The USGS gemstones list puts chalcedony and jasper under cryptocrystalline varieties of silica in its quartz entry, with agate, onyx, sardonyx, bloodstone, carnelian, chrysoprase, moss agate, plasma, prase and sard. The International Gem Society defines chalcedony as any form of microcrystalline or cryptocrystalline quartz, and gives its variety-of field as quartz.
So the label is a texture name. A specimen labelled chalcedony is a specimen of quartz; whether a crystal-free quartz is still a mineral is answered on is quartz a mineral, and the separation from crystal quartz is on is chalcedony quartz.
What is chalcedony made of? Quartz, moganite and water, worked
The study. Monico and co-authors, Mineralogical Magazine 88 (2024), from the University of Milan with the Istituto Gemmologico Italiano, describe chalcedony as microcrystalline fibrous quartz of alpha-quartz crystallites 50 to 100 nm across, in a fibre-like texture. Length-fast chalcedony has the quartz c-axis across the fibre; length-slow chalcedony, or quartzine, along it. They give chalcedony 1 to 2 wt% water, as molecular H2O and silanol groups, and mogánite at 5 to 20 wt%.
The 78-to-94 rule, worked. At the low end, 100 − 5 mogánite − 1 water leaves 94 wt% alpha-quartz; at the high end, 100 − 20 − 2 leaves 78 wt%. Trace elements and inclusions take a little more. By Raman spectroscopy the paper found alpha-quartz and mogánite in all three varieties it studied: agate, chrysoprase and a new bluish-green chalcedony it names aquaprase.
Mogánite is a mineral in its own right: the IMA list gives it SiO2·nH2O, status Rn, 1999-035, Spain. The paper describes it as monoclinic, built of left- and right-handed quartz lamellae alternating at the unit-cell scale. Opal, the other hydrated silica row on the list, is a different material; see opal.
Agate, onyx, carnelian, chrysoprase and jasper: names within chalcedony
The gem society’s definitions. Agate is chalcedony distinguished by pattern, translucency or moss-like inclusions. Onyx has straight, nearly parallel bands; the gem society notes that some references instead treat onyx as a kind of agate. Carnelian is red to orange-brown chalcedony coloured by iron. Jasper is opaque, solid or patterned cryptocrystalline quartz. Chrysoprase is translucent green chalcedony coloured by nickel.
A colour cause with a mineral behind it. In chrysoprase, Monico and co-authors identified willemseite, a nickel-bearing layered silicate, and say it is responsible for the green. Willemseite has its own IMA row, Ni3Si4O10(OH)2, status A, 1971 s.p., South Africa.
Treated colour. The gem society says chalcedony is porous and that dyeing is ancient and common; black onyx is usually dyed; almost any chalcedony can be turned red by heating, and carnelian should be assumed heated unless you are told otherwise. Checks are on how to tell if a crystal is dyed. “Mexican onyx” is banded calcite, not chalcedony.
How heavy is chalcedony? Density against crystal quartz
On the sheet. The Handbook quartz sheet gives D 2.65 for quartz and 2.59 to 2.63 when massive. Worked as 20 − 20/D, a 20.00 g piece reads 12.45 g in water as crystal quartz, the figure on the live quartz page, and 12.28 to 12.40 g as massive quartz.
On the gem society’s figures. Agate 2.60 to 2.64 reads 12.31 to 12.42 g; carnelian 2.58 to 2.64, 12.25 to 12.42 g; onyx 2.651, 12.46 g. Its chalcedony panel gives the wide range 2.55 to 2.91, 12.16 to 13.13 g, which also covers jasper and its impurities. The scale shows chalcedony as quartz-weight; it cannot tell chalcedony from crystal quartz. Method: how to measure specific gravity at home.
The quotable line: a chalcedony reading 12.25 to 12.46 g in water for 20.00 g is behaving as quartz, because it is quartz. Opal, at GIA’s 2.15, reads 10.70 g and sits well clear.
Where chalcedony forms, and the Handbook sheets that name it
Settings. Monico and co-authors say chalcedony forms most favourably at low temperature near the surface, typically in sedimentary rocks where it precipitates in pores and fractures from silica-rich solutions, and also through hydrothermal activity and in volcanic cavities. The gem society lists sources for chalcedony as California, Iceland, India, Mexico, Namibia, Nevada, Oregon and Russia, and for carnelian Brazil, Egypt, India and Uruguay.
On the sheets. The calcite sheet names chalcedony among associates in vesicles, with zeolites; the cinnabar and metacinnabar sheets list “chalcedony” in quotation marks; the stibnite sheet lists chalcedonic quartz. See calcite and cinnabar.
On this site. Related silica pages are quartz and opal, and the full list is in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.