Is onyx a mineral?
Onyx is not a mineral species but banded chalcedony, which is quartz, hardness 7. Onyx marble is calcite, hardness 3. Dyed black onyx has SG 2.57 to 2.64.
Not as a species. Onyx is chalcedony with straight, parallel bands, and chalcedony is quartz, SiO2, hardness 7; the IMA list has no onyx row. “Onyx marble” is banded calcite, hardness 3. Dyed black onyx reads 12.22 to 12.42 g in water for 20.00 g; of 36 black look-alikes GIA lists, only glass overlaps.
In short
- A variety name for a mineral. The International Gem Society onyx article calls onyx a type of chalcedony, itself a form of microcrystalline quartz, with straight, nearly parallel bands. Its panel gives SiO2, hardness 7, SG 2.651. A whole-word script finds no onyx line on the September 2026 IMA-CNMNC Master List; quartz has one. So onyx is not a mineral species, but true onyx is made of one: quartz.
- Three definitions, one source. IGS’s onyx article defines onyx by straight bands of any colour and notes that some references treat it as a black, brown or white banded agate; its chalcedony article says straight bands, with black onyx black and white; its quartz article says banded black and white chalcedony. All three agree on chalcedony and bands; they disagree on colour.
- Not marble. IGS says “Mexican onyx”, “limestone onyx” or “onyx marble” is a type of banded calcite from limestone caves, with perfect cleavage and hardness 3 against onyx’s 7. Calcite is its own IMA species. Documented failure of the trade name: IGS captions a Mixtec monkey vessel at the Metropolitan Museum of Art as onyx marble; the Metropolitan Museum of Art’s own record for the vessel gives its medium as travertine.
- The 2.57 to 2.64 window. GIA’s Gems & Gemology black-opaque study (Winter 1996) gives dyed black chalcedony SG 2.57 to 2.64, so a 20.00 g piece reads 12.22 to 12.42 g in water. Its Table 1 prints an SG for 36 other black opaques; a script finds only glass (2.51 to 3.21) overlapping. GIA adds the caveat that SG is less reliable for aggregates and porous materials, which chalcedony is.
- Onyx and obsidian are not the same. IGS says “onyx obsidian” is banded volcanic glass and “onyx opal” is layered common and precious opal: names borrowed for the look. Whether obsidian is a mineral is on is obsidian a mineral; it is not repeated here.
| Material | What it is (source) | Hardness | Specific gravity | 20.00 g piece in water | Inside the window? |
|---|---|---|---|---|---|
| Dyed black chalcedony (“black onyx”) | Treated chalcedony, quartz (GIA) | 7 (IGS onyx panel) | 2.57 to 2.64 (GIA) | 12.22 to 12.42 g | The window |
| Onyx, banded | Straight-banded chalcedony (IGS) | 7 | 2.651 (IGS) | 12.46 g | 0.03 g above |
| Quartz, massive | SiO2, IMA species (Handbook) | 7 | 2.59 to 2.63 | 12.28 to 12.40 g | Yes |
| Glass | Manufactured (GIA) | — | 2.51 to 3.21 (GIA) | 12.03 to 13.77 g | Yes: the only GIA overlap |
| Labradorite feldspar | Black opaque (GIA) | — | 2.69 (GIA) | 12.57 g | No: 0.14 g above |
| Dolomite/quartzite rock | Aggregate (GIA) | — | 2.74 (GIA) | 12.70 g | No |
| Dolomite | Black opaque (GIA) | — | 2.8 to 2.9 (GIA) | 12.86 to 13.10 g | No |
| Onyx marble | Banded calcite (IGS); Ca(CO3) (IMA) | 3 | 2.7102 (Handbook) | 12.62 g | No: 0.20 g above |
| Aragonite (comparator only) | Ca(CO3), the other IMA row (Handbook) | 3.5 to 4 | 2.95 | 13.22 g | No |
| Jet | Black opaque (GIA) | — | 1.20 to 1.30 (GIA) | 3.33 to 4.62 g | No |
Is onyx a mineral or a rock?
Neither, as a name; a mineral, as a material. A mineral species is what the IMA list names. A whole-word scan of the September 2026 list finds no line containing onyx, sardonyx, chalcedony, agate or marble. It does have quartz, SiO2, status A, 1967 s.p. The Handbook of Mineralogy quartz sheet lists fine-grained to microcrystalline (“chalcedony”) among quartz’s habits. The International Gem Society calls onyx a type of chalcedony, and chalcedony a form of microcrystalline quartz; its onyx panel gives SiO2, hexagonal (trigonal), microcrystalline, hardness 7, SG 2.651, RI 1.544 to 1.553, the same refractive indices the quartz sheet prints for ω and ε.
So the label is two-part. True onyx is quartz in a banded, microcrystalline form, so the specimen is a mineral even though onyx is not a species. Is it a crystal? Not one you can see: IGS calls it microcrystalline. The distinction between a crystal and a mineral is on crystal vs mineral: what is the difference. Whether chalcedony counts as quartz, and how to choose between those two words, is on is chalcedony quartz and is not repeated here.
Which colour is onyx? Three IGS definitions
The bands are agreed; the colour is not. The International Gem Society onyx article gives two prevalent views: some mineralogical references treat onyx as a variety of agate, black, brown or white banded, while IGS treats agate and onyx as banded chalcedony told apart by the bands, curved in agate, straight or nearly parallel in onyx, regardless of colour. The International Gem Society chalcedony article says chalcedony with straight bands of colours, with black onyx black and white. The International Gem Society quartz article says banded black and white chalcedony.
The named kinds. IGS calls black-based stones with a white upper layer “true” or Arabic onyx; white with reddish, brownish or yellow layers sardonyx; red-based with a white layer cornelian or carnelian onyx; and a very thin white layer over black, which can look blue, nicolo onyx. Whether banded carnelian is an agate is on is carnelian an agate.
Black onyx is mostly dyed. IGS says onyx typically forms in very thin layers, only rarely thick enough to cut a solid colour, and that most solid black pieces sold today are dyed black chalcedony. GIA’s 1996 study calls the material treated black chalcedony (“black onyx”) and notes the black chalcedony may have been a banded agate before treatment. Dye checks are on how to tell if a crystal is dyed.
Is onyx a type of marble?
No: onyx marble is a different mineral. IGS says “Mexican onyx”, “limestone onyx” or “onyx marble” is a type of banded calcite found in limestone caves, that calcite is a distinct species and not a variety of chalcedony, and that calcites have perfect cleavage and much lower hardness: 3 against 7. The Handbook of Mineralogy calcite sheet gives calcite hardness 3, perfect {10-11} cleavage and density 2.7102. The IMA list gives calcite Ca(CO3), status G, 1836. A steel point decides between the two; see how to tell calcite from quartz and how to test mineral hardness properly.
Documented failure: the museum label. IGS illustrates onyx marble with a Mixtec monkey vessel in the Metropolitan Museum of Art, captioned “onyx marble,” known as tecali, Mexico, 10th to 12th century. The museum’s own record, fetched from its collection API, gives the medium as travertine, pyrite, shell, and the date 1350 to 1521 CE. The two sources disagree on both the stone name and the date. Trade names for banded carbonate are not a mineral identification; this page uses the IMA species, calcite, and claims nothing about aragonite in such material, because no GIA, IGS or museum source fetched for it says so. Aragonite, the other Ca(CO3) row, appears in the table only as a comparator.
How do you tell onyx from black look-alikes? The 2.57 to 2.64 window
The rule. GIA’s Table 1 gives dyed black chalcedony (“black onyx”) SG 2.57 to 2.64 and RI 1.530 to 1.539. A 20.00 g piece therefore reads between 12.22 and 12.42 g suspended in water. A script over the table’s 36 other entries with a printed SG finds one that overlaps, glass at 2.51 to 3.21. The nearest outside are labradorite at 2.69 (12.57 g), dolomite/quartzite rock at 2.74 and dolomite at 2.8 to 2.9; jet, plastics, black coral and cubic zirconia are far off. A reading outside the window rules out dyed chalcedony; a reading inside narrows it to chalcedony or glass. Banded natural onyx at IGS’s 2.651 reads 12.46 g, just above it. The method is on how to measure specific gravity at home.
Where the window fails. GIA says specific gravity is somewhat less reliable for aggregates and porous materials, and hydrostatic measurements less precise than refractive index; chalcedony is both an aggregate and porous. GIA also says that in most cases advanced techniques, such as X-ray diffraction, are needed to identify a black opaque, and may not be conclusive. IGS names black devitrified glass as a common onyx substitute, so the one overlap is the one that matters; bubbles and swirl marks are covered on how to tell if a crystal is fake. Other guides are under the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.