Marble, quartzite and onyx are commercial names before they are geological ones, and the difference matters because each sits under a different ASTM specification with different numbers behind it. Marble is specified by ASTM C503/C503M and may absorb no more than 0.20% water by weight. Quartzite is specified by ASTM C616/C616M and is held to a minimum compressive strength of 20,000 lbs/in² — nearly three times the 7,500 lbs/in² minimum for marble. Onyx has no standard of its own: the trade files it with marble. Below is what each word on a spec sheet is actually promising, and the two checks that settle an argument in a slab yard.
Two naming systems, running at once
A geologist names a rock by how it formed; the stone trade names it by how it behaves. The two systems overlap, but not reliably, and the Natural Stone Institute is explicit that the commercial grouping is the looser — dense limestones that take a polish are routinely sold within the marble group because they “appear, behave and perform more similarly to marble than to limestone.”
That is not deception; it is a classification built around performance in a building. The problem starts only when a name travels further than the properties behind it — which is what has happened to quartzite.
Marble: ASTM C503, and the 0.20% number
Commercially, marble is any crystallised carbonate rock that takes a polish. Geologically, true marble is metamorphosed limestone — calcite or dolomite recrystallised under heat and pressure, which is why veining in a Carrara block runs continuously through the stone rather than sitting on its face.
ASTM C503/C503M sets the floor. As tabulated in the Natural Stone Institute’s Dimension Stone Design Manual (2022), the requirements are:
- Absorption by weight: 0.20% maximum (ASTM C97). Measured marbles run 0.060–1.0%.
- Density: 162 lb/ft³ minimum for calcite marble, 175 lb/ft³ for dolomite (ASTM C97).
- Compressive strength: 7,500 lbs/in² minimum (ASTM C170), against a measured range of 6,000–35,000.
- Modulus of rupture: 1,000 lbs/in² minimum (ASTM C99).
- Abrasion resistance: Ha 10 minimum (ASTM C241/C1353), rising to 12 for commercial floors.
The absorption figure is the one worth carrying into a bathroom. A stone taking up two parts of water in a thousand is not porous in any ordinary sense; what makes marble demanding is chemistry, not absorption. Calcite reacts with acid — that is the whole of the etching story, and the basis of the field test below.
Quartzite: the most mislabelled word on the sheet
True quartzite is sandstone whose quartz grains have been fused by heat and pressure into an interlocking crystalline mass. ASTM C616/C616M covers quartz-based stone in three grades, and the gap between them is wide: sandstone at 4,000 lbs/in² compressive and 8.0% maximum absorption, quartzitic sandstone at 10,000 lbs/in² and 3.0%, and quartzite at 20,000 lbs/in² and 1.0%. Only the last of those is what a buyer means by the word.
The industry has been direct about what goes wrong here. The Natural Stone Institute — then MIA+BSI — devoted a technical bulletin to it:
“The problem with quartzite stems from the fact that it is commonly mislabeled. Some quartzite is the real deal, but sometimes marble or dolomitic marble are labeled as quartzite.”
— MIA+BSI (Natural Stone Institute), Technical Bulletin, Quartzite Need Not be a Confusing Stone, Vol. VIII, Issue I, June 2017
Its conclusion is that quartzite itself is consistent — “quartzite has very consistent properties” — and that the inconsistency lives in the labelling. A buyer paying a quartzite premium for maintenance-free behaviour, and receiving dolomitic marble, has bought a stone that will etch.
Onyx: real stone, no standard of its own
Onyx is the outlier: not metamorphic at all, but a chemical sedimentary stone formed as dripstone. The Dimension Stone Design Manual is plain about it — “Onyx originates in the dripstone deposits of limestone caverns,” and “tends to be more crystalline, strongly banded and colored in browns to yellows and clear.” That banding and translucency is the point of it; backlit onyx is a different material experience from any marble.
Having no specification of its own, it borrows one: the manual records that “the dimension stone industry traditionally includes other types of stone, such as onyx, in the same classification as marble.” That means a C503 test report and the same calcite chemistry — onyx etches. It is also sold as travertine, though “commercial practice generally distinguishes the two stones.” Travertine has its own specification, ASTM C1527.
The spec sheet, side by side
| Marble | Quartzite | Onyx | |
|---|---|---|---|
| ASTM specification | C503/C503M | C616/C616M, Quartzite grade | None; filed under C503 by trade practice |
| How it formed | Metamorphosed limestone | Metamorphosed sandstone | Chemical sediment — cave dripstone |
| Principal mineral | Calcite / dolomite | Quartz (Mohs 7) | Calcite / aragonite |
| Max absorption | 0.20% | 1.0% | Tested as marble |
| Min density | 162 lb/ft³ (calcite) | 160 lb/ft³ | Tested as marble |
| Min compressive strength | 7,500 lbs/in² | 20,000 lbs/in² | Tested as marble |
| Min modulus of rupture | 1,000 lbs/in² | 2,000 lbs/in² | Tested as marble |
| Etches in household acid | Yes | No | Yes |
Figures: ASTM C503/C503M and C616/C616M requirements as tabulated in the Natural Stone Institute, Dimension Stone Design Manual (2022), chapters on Marble and Onyx and on Quartz-Based Stone.
Two field checks that settle it
You do not need a laboratory to separate these three in a slab yard. The NSI bulletin gives two tests.
The acid drop. The standard version “calls for a 5% to 10% solution of hydrochloric acid” — one drop on an inconspicuous edge. “If bubbles are present and easy to see, then the stone contains calcite.” Marble and onyx fizz; quartzite does not. Household vinegar works more slowly: leave it “15 minutes or so” and marble shows obvious etching, dolomitic marble a subtler mark, quartzite “zero etching.”
The glass scratch. Quartz is 7 on the Mohs scale, well above window glass at roughly 5.5. “Real quartzite will easily scratch the glass,” the bulletin states, while “non-quartzite will either leave no scratch or a very faint scratch.” Calcite, the mineral in marble and onyx, sits at 3 — the Mohs scale’s own index mineral for that number, and soft enough to be marked by a steel blade.
What this means for a basin
For a carved basin rather than a worktop, the ranking inverts. Quartzite’s 20,000 lbs/in² is an advantage in a thin slab spanning a cabinet; it is close to irrelevant in a basin wall 30–40 mm thick carved from one block, where absorption and modulus of rupture govern and marble clears both. What that hardness costs is carving: an interlocking quartz matrix at Mohs 7 does not take a hand-cut flute the way calcite does.
That is why hand-carved basins are overwhelmingly marble, travertine and onyx — the carbonate stones. The honest trade-off is etching, which we treat as patina rather than damage, and which a sealer slows but does not prevent.
Two basins, and what their spec sheets say
More by material: Carrara basins, travertine basins, above-counter vessel basins, or the full stone basin collection. If the stone you want is named on a drawing rather than in a catalogue, that is what the custom order page is for — name the stone, the finish and the dimensions, and we will tell you what it carves like before anything is cut.
Frequently asked questions
What is the difference between marble and quartzite?
Marble is metamorphosed limestone, built from calcite or dolomite; quartzite is metamorphosed sandstone, built from quartz. They sit under different ASTM specifications with different floors: marble under C503/C503M at 0.20% maximum absorption and 7,500 lbs/in² minimum compressive strength, quartzite under C616/C616M at 1.0% absorption and 20,000 lbs/in². The practical difference is chemical, not structural — calcite reacts with household acids and etches, quartz does not.
Is onyx a type of marble?
Not geologically. Onyx is a chemical sedimentary stone that, as the Natural Stone Institute’s Dimension Stone Design Manual puts it, “originates in the dripstone deposits of limestone caverns” — it never underwent metamorphism. Commercially it is treated as marble: the manual records that the industry “traditionally includes other types of stone, such as onyx, in the same classification as marble,” so onyx is tested and specified under ASTM C503. It shares marble’s calcite chemistry and therefore etches the same way.
How can I tell if a slab labelled quartzite really is quartzite?
Two tests, both from the Natural Stone Institute’s quartzite technical bulletin. Put one drop of 5–10% hydrochloric acid on an inconspicuous edge: visible bubbles mean the stone contains calcite, so it is marble or dolomitic marble, not quartzite. Then try to scratch a piece of glass with a sharp corner of the sample — quartz is Mohs 7 and “real quartzite will easily scratch the glass,” while non-quartzite leaves no scratch or a very faint one. Household vinegar left 15 minutes is a slower substitute for the acid drop.
Does “commercial marble” mean the stone is not real marble?
No — it means the name is being used in its trade sense rather than its geological one. The commercial marble group covers crystallised carbonate rocks that take a polish, including dense limestones that the Natural Stone Institute notes “appear, behave and perform more similarly to marble than to limestone.” Ask for the test report rather than the label: C503 absorption, density, compressive strength and modulus of rupture tell you how the stone will behave.
