Is chalcedony quartz?
Chalcedony is quartz with crystals too small to see: hardness 7, SG 2.55 to 2.91 (IGS), no IMA row. Mexican onyx is calcite, hardness 3, not chalcedony.
Yes. Chalcedony is quartz, SiO2, in crystals too small to see: hardness 7 and specific gravity 2.55 to 2.91 on the International Gem Society's figures, with no IMA row of its own. A 2024 study adds 5 to 20 wt% moganite. Choose the word chalcedony for crystal-free, waxy quartz; Mexican onyx is calcite, hardness 3.
In short
- Chalcedony is quartz. The Handbook quartz sheet uses the word chalcedony, in quotation marks, for quartz’s fine-grained to microcrystalline habit. A script over the September 2026 IMA-CNMNC Master List finds a row for quartz and none for chalcedony, agate, onyx or jasper.
- Mostly quartz, not only quartz. Monico and co-authors, Mineralogical Magazine 88 (2024) put mogánite, a separate silica mineral, at 5 to 20 wt% of chalcedony and water at 1 to 2 wt%. The chemistry is worked on chalcedony; for labelling, chalcedony is a form of quartz.
- The visible-crystal rule. If you can see crystal faces or grains, call it quartz, or its variety name. If it is translucent to opaque, waxy to dull and shows no crystals at 10x, call it chalcedony; banded, agate or onyx; opaque and patterned, jasper. Hardness and density do not choose between them: both are 7, and a 20.00 g piece reads 12.45 g in water as crystal quartz against 12.25 to 12.46 g as agate, carnelian or onyx.
- Where the rule fails: Mexican onyx. The International Gem Society says Mexican, limestone or marble onyx is banded calcite. On the scale, onyx at 2.651 reads 12.46 g and calcite at 2.7102 reads 12.62 g, 0.16 g apart. Hardness decides: calcite 3, chalcedony 7. See how to tell calcite from quartz.
- Not opal, not glass. Opal is SiO2·nH2O, hardness 5 to 6.5, specific gravity 2.15 on GIA’s figures, and a 20.00 g piece reads at most 11.11 g in water. Glass ranges 11.30 to 15.56 g on the gem society’s figures and overlaps chalcedony; bubbles and swirls catch it.
| Name on the label | What it is (source) | Visible crystals? | Lustre | Hardness | Specific gravity | 20.00 g piece in water | Choose this name when |
|---|---|---|---|---|---|---|---|
| Quartz (crystal) | SiO2, IMA row (Handbook sheet) | Yes | Vitreous (sheet) | 7 | 2.65 | 12.45 g | Faces or grains are visible |
| Chalcedony | Microcrystalline quartz with moganite (IGS; Monico 2024) | No | Waxy to dull (IGS; sheet, massive) | 7 | 2.55 to 2.91 (IGS) | 12.16 to 13.13 g | Crystal-free, translucent, unpatterned |
| Agate | Banded or patterned chalcedony (IGS) | No | Greasy, waxy (IGS) | 6.5 to 7 | 2.60 to 2.64 (IGS) | 12.31 to 12.42 g | Curved or concentric bands, or moss |
| Onyx | Chalcedony with straight bands (IGS) | No | Waxy (IGS) | 7 | 2.651 (IGS) | 12.46 g | Straight, parallel bands |
| Jasper | Opaque cryptocrystalline quartz (IGS) | No | Waxy to dull (IGS) | 7 (IGS) | 2.55 to 2.91 (IGS) | 12.16 to 13.13 g | Opaque, solid or patterned |
| Mexican onyx | Banded calcite (IGS); CaCO3 (Handbook sheet) | Sometimes | Vitreous; pearly on cleavages (calcite sheet) | 3 | 2.7102 (sheet) | 12.62 g | Never chalcedony: a steel point marks it |
| Opal | SiO2·nH2O, IMA row (GIA; IMA list) | No | Vitreous, waxy, pearly (IGS) | 5 to 6.5 (GIA) | 2.15 (GIA) | 10.70 g | Play-of-colour, or under 11.11 g |
| Glass | Manufactured (IGS glass article) | No | Vitreous (IGS) | 5 to 6 (IGS) | 2.30 to 4.50 (IGS) | 11.30 to 15.56 g | Bubbles, swirl marks, mould marks |
Is chalcedony quartz? The criteria
Three tests decide it, in this order. Composition: chalcedony is SiO2, the formula of quartz, with 1 to 2 wt% water. Structure: Monico and co-authors, Mineralogical Magazine 88 (2024) describe it as fibres of alpha-quartz crystallites 50 to 100 nm across, which is to say quartz, with 5 to 20 wt% of a second silica mineral, mogánite. Name: the September 2026 IMA-CNMNC Master List has no chalcedony row, and the Handbook quartz sheet uses chalcedony for a habit of quartz.
On all three, chalcedony is quartz. The gem society’s chalcedony article gives it the formula SiO2, hexagonal (trigonal) microcrystalline, hardness 7, and records its variety-of field as quartz. The USGS lists it under the cryptocrystalline varieties of silica in its quartz entry.
What this page does not repeat. The clause-by-clause mineral test is on is quartz a mineral; the composition arithmetic, 78 to 94 wt% alpha-quartz, is on chalcedony. The identification hub is the collecting guides.
Chalcedony vs quartz crystal: the visible-crystal rule
The visible-crystal rule: if you can see the crystals, call it quartz; if a 10x lens shows none, call it chalcedony. The gem society defines chalcedony as quartz whose crystals are too small to be seen without high magnification, and separates crystalline quartz, amethyst, smoky quartz, citrine, rose and milky quartz, from cryptocrystalline quartz. The Handbook quartz sheet gives quartz a vitreous lustre, waxy to dull when massive.
Why the usual tests do not choose. Hardness: the quartz sheet gives 7 and the gem society 7 for chalcedony. Density: crystal quartz 2.65 reads 12.45 g in water for a 20.00 g piece, and agate, carnelian and onyx read 12.25 to 12.46 g on the gem society’s figures. Refractive index: the quartz sheet prints 1.544 and 1.553; the gem society gives chalcedony 1.540 to 1.553 and agate 1.530 to 1.543. These overlap or touch. The lens is the instrument that decides.
Then the variety name. Banded in curves: agate. Straight bands: onyx. Opaque, solid or patterned: jasper. Red to orange: carnelian. Translucent green: chrysoprase. Colours may be dyed; see how to tell if a crystal is dyed.
Where the rule fails: Mexican onyx, opal and glass
Mexican onyx is the documented failure. The gem society’s onyx article says Mexican, limestone or marble onyx is banded calcite from limestone caves, with perfect cleavage and hardness 3. It can look crystal-free and banded, so the visible-crystal rule would call it onyx. Weight barely helps: 12.62 g in water for calcite against 12.46 g for onyx. A steel point does: it marks calcite and not chalcedony. See how to test mineral hardness properly and how to tell calcite from quartz.
Opal. Also silica, also crystal-free, but SiO2·nH2O with up to 20% water, hardness 5 to 6.5 and specific gravity 2.15 on GIA’s figures. A 20.00 g piece reads 10.70 g, under the 11.11 g ceiling set out on opal. The gem society’s onyx opal is layered opal, not onyx. Whether opal is a mineral is on is opal a mineral.
Glass. The gem society gives glass 2.30 to 4.50, 11.30 to 15.56 g, which covers chalcedony, and hardness 5 to 6; it names black devitrified glass as an onyx substitute. Bubbles and swirl marks under the lens are the check; see how to tell if a crystal is fake.
Which label is wrong for which piece
Quartz is correct for chalcedony, agate, onyx, jasper and carnelian: they are quartz. It is wrong for Mexican onyx, which is calcite, and for opal.
Chalcedony is wrong for a single quartz crystal or a druse of visible points, and for opal, calcite and glass. It is incomplete, though not wrong, for a mogánite-rich piece; measuring that share took X-ray and Raman work in the 2024 study. Agate is wrong for unbanded, pattern-free material, and onyx for anything that a steel point marks.
On a label, write what the test showed: quartz, variety chalcedony (agate), when bands are seen; quartz (chalcedony) when not. Nothing on this site is offered for sale; the wanted list is the only commercial route.