Quartzite is suddenly everywhere. Is it true that so many quarries have recently sprung up, and everything is quartzite? Or is it just that "quartzite" sells better than the truth? You've been told a slab is quartzite, but you're not sure that's true. Other suppliers offer the same stone as marble, some as dolomite, others as "soft quartzite" — with most simply labelled "quartzite."
You're left holding a sample with no way to tell which one you've got. A lab report is the only fully conclusive answer. What if a report isn't available, you've already picked the slab, and you've imagined it in your space — how can it be tested? There is no single test that will settle it. Each test is discussed in turn, along with what it proves and where its evidence ends — as well as the specific stones (Super White, Fantasy Brown, Taj Mahal) that appear in genuine buyer disputes, and the ASTM numbers that settle it when a home test fails.
How can I tell if quartzite is real or mislabeled?
You cannot settle it with a single test, and any one test used alone will mislead you. Three different rocks reach the slab yard under the word quartzite — genuine quartzite, marble (usually dolomitic marble) and sandstone — and each home test rules out one of them without confirming the others.
That is the whole problem in one sentence, and it is why so many buyers run a test, get the result they hoped for, and are still wrong.
The three rocks are genuinely different materials. ASTM C119, the terminology standard that both the quartzite and marble specifications defer to, separates them by how they break: sandstone fractures around its grains, quartzitic sandstone may fracture around or through them, and quartzite fractures conchoidally through the grains, because metamorphism has fused grain and cement into one mass. Marble is not in that family at all — C119 defines it as a carbonate rock recrystallised by heat and pressure, composed principally of calcite and dolomite.
So the practical sequence is: use the glass test to eliminate marble, use the acid test to detect carbonate, use etching history as corroboration, and use a laboratory when the answer has to hold. The sections below take each test in turn, and each one ends where its evidence actually ends rather than where a sales conversation would like it to.
Will a glass scratch test prove it's quartzite?
No. It proves the slab is not marble, and that is all it proves. Both genuine quartzite and quartzitic sandstone are built from quartz, which sits at 7 on the Mohs scale against roughly 5.5 for window glass — so both scratch glass, and passing the test tells you nothing about which of the two you are holding.
What it does eliminate is the carbonate family. Calcite is Mohs 3 and dolomite 3.5 to 4 (Kansas Geological Survey), both far below glass, so a marble slab will not scratch it. One qualifier has to ride along. The Natural Stone Institute records that some marbles carry minor quartz and scratch glass in patches — of Super White it warns, "You may find that there are some areas that scratch glass and others that don't." A scratch in one spot on a variegated slab proves nothing. A slab that bites consistently across the whole face has cleared the most likely failure.
Two cautions on the number itself. Glass hardness is not a constant — Geology.com notes real glass items test anywhere from about 4 to 7 — so treat 5.5 as the conventional field figure rather than a measured threshold. And check the mark before you believe it: the Natural Stone Institute notes that a non-quartzite dragged across glass feels slippery and leaves a powdery trail that rubs right off — it looks exactly like a scratch until you wipe it.
Used honestly, the glass test is a first filter. It is not an identification.
What does the acid test actually prove?
It detects carbonate. It does not identify marble, and it does not separate quartzite from sandstone. A drop of dilute acid — the standard geologic reagent is a 5% to 10% hydrochloric solution, per the Natural Stone Institute, with Building Stone Magazine specifying 10% — will fizz on calcite and stay inert on quartz.
Three limits decide whether that result means anything.
- Calcite-cemented sandstone reacts too. A fizz proves carbonate is present, not that the slab is marble. A sandstone held together by calcite cement will bubble while remaining, unambiguously, a sandstone.
- Dolomitic marble may not visibly react at all. This is the failure that matters most, because dolomitic marble is the stone most often sold as quartzite. The Natural Stone Institute is explicit that the powdered-rock acid test is the only way to tell dolomitic marble from calcite marble: scrape a little powder from the stone, apply acid to the powder, and inspect the drop under a magnifying glass — NSI is explicit that the bubbling can be too faint to catch with the naked eye, which is exactly how dolomitic marble passes as quartzite. On an intact polished face, dolomite can pass for inert.
- Run it on an offcut or a hidden edge. Acid on a finished face can leave a permanent dull mark. Test the underside, a scrap from the fabricator, or the back of a sample — never the visible surface of a slab you have already paid for.
Looking for Genuine Quartzite?
Every slab we sell as quartzite is classified against ASTM C616 before it's listed â not sold on a trade name alone. See what's currently in stock, with the classification stated upfront.
View Genuine Quartzite StockA no-fizz result on a polished top, then, is close to meaningless on its own. A no-fizz result on powdered stone is worth something.
Why is my "quartzite" countertop etching?
Because it contains carbonate, and genuine quartzite does not. Etching is not staining and not poor sealing — it is a chemical reaction in which acid dissolves calcite or dolomite, leaving a dull mark that no sealer prevents and no cleaner removes. Quartz does not react, so a countertop that etches is telling you what it is made of.
This is the single most common way buyers discover a mislabelled slab, and it usually arrives months after installation via lemon juice, wine, vinegar or a bathroom cleaner. Houzz threads on the pattern run for years: one owner posting under Help with etching on quartzite! drew the blunt reply, “Quartzite does NOT etch. Marble etches. You do not have granite or quartzite. You have marble.”
Sealing is worth understanding here, because it is the usual reassurance offered and it does not apply. A sealer resists liquid absorption and so helps with staining; etching happens at the surface, above the sealer, where acid meets mineral. Use Natural Stone, the Natural Stone Institute's consumer publication, puts it plainly: "Sealers do not protect a stone from interactions with acid." One fabricator's published comparison, from Italian Marble & Granite, exposed sealed and unsealed halves of several slabs to lemon juice for 45 minutes and found the sealed halves of its marble samples etched anyway.
If a surface sold to you as quartzite is etching from ordinary kitchen acids — lemon, wine, vinegar, a bathroom cleaner — the slab is not quartzite. One qualifier belongs on that: NSI notes that strong acids, such as those in rust-stain removers, will etch even granite and quartzite, so the rule holds for household acids and not for everything under the sink. Within that limit the conclusion is reliable in a way that no home test is.
Is there such a thing as "soft quartzite"?
No. "Soft quartzite" has no geological meaning, and the two industry publications that have addressed it say so plainly — though both, as the source list shows, were written by the same geologist. It is a trade term, and it exists to explain away a stone that is not behaving like the material it was sold as.
The Natural Stone Institute devotes a section of its quartzite bulletin to the point: "There is no such thing as soft quartzite. There is only one kind of quartzite and it's hard and will not etch." Writing in Building Stone Magazine, geologist Karin Kirk is sharper still — she calls it "a fictional term created to explain why a mislabeled quartzite doesn't act like a real quartzite," adds that "soft quartzite is, in fact, marble," and treats the phrase as a red flag that the person using it does not understand the material.
For a buyer the practical value is high, because the term usually surfaces at a diagnostic moment: after a slab has etched, or when a hardness question is being deflected. A Houzz thread titled Kitchen Counter — Quartzite v. Soft Quartziteshows the pattern exactly, with a fabricator replying that there is no such thing and that the stone in question is dolomitic marble.
Treat the phrase as information about the seller, not about the stone.
How much quartz does a stone need before it can be called quartzite?
A measured free-silica content of at least 95%. ASTM C616/C616M-22, the specification for quartz-based dimension stone, sorts the whole family on that one number: Class I Sandstone at 60% minimum free silica, Class II Quartzitic Sandstone at 90%, and Class III Quartzite at 95%. Free silica, in the standard's own footnote, means detrital quartz grains plus authigenic silica — the cement counts.
The physical minimums tighten across the same three classes:
| Property (test method) | Sandstone | Quartzitic sandstone | Quartzite |
|---|---|---|---|
| Absorption, max (C97) | 8% | 3% | 1% |
| Density, min (C97) | 125 lb/ft³ | 150 lb/ft³ | 160 lb/ft³ |
| Compressive strength, min (C170) | 4,000 psi | 10,000 psi | 20,000 psi |
| Modulus of rupture, min (C99) | 350 psi | 1,000 psi | 2,000 psi |
For comparison, ASTM C503/C503M-23 allows marble a maximum absorption of 0.20% — five times tighter than quartzite's 1% ceiling, and forty times tighter than sandstone's 8%. Low absorption is therefore not evidence of quartzite. That same standard sorts dimension marble into exactly two classes, I Calcite and II Dolomite, which settles a common confusion: the dolomitic marble at the centre of this problem is a marble by standard, not something in between.
Two honest caveats. C616 states the 60/90/95 thresholds but references no test method for measuring silica content, so the figure comes from petrography rather than from the specification itself. And C119's commercial definition puts quartzite above 17,000 psi where C616 requires 20,000 — the two standards do not agree, and a report citing one is not citing the other.
Cristallo Quartzite â See the Classification
We supply Cristallo as quartzite, sourced and classified the same way this article recommends checking any slab â not sold on the name alone. See current lots and the classification behind them.
View Cristallo QuartziteHow do I tell quartzite from sandstone at the slab?
By how it breaks, not by how it scratches. Both rocks are quartz, so both bite into glass; the difference shows on a fresh broken edge, where quartzite fractures through its grains and ordinary sandstone fractures around them, leaving the grains whole.
That criterion is published in both systems. ASTM C119uses it to draw its three scientific definitions, and the peer-reviewed statement of it is Howard 2005, which identifies field quartzite as a quartz-rich rock that fractures irregularly through both grains and cement to an irregular or conchoidal surface, and separates it from quartzose sandstone, which is softer and fractures around individual grains.
Two limits, and the second is the one that matters commercially. The test needs a genuinely broken edge — not a polished face, not an epoxied one. And it does not prove metamorphism: Howard splits quartzite into orthoquartzite and metaquartzite, sedimentary and metamorphic, and separates those only by microscopic texture. Writing for Use Natural Stone, Karin Kirk puts the gradient plainly — partly metamorphosed stones show grains that "are still visible," and "there is no exact moment that sandstone becomes quartzite."
Where the edge is inconclusive, the number decides: C616 allows quartzite 1% absorption and quartzitic sandstone 3%.
Can I check absorption myself?
No, and the gap between the real method and a kitchen-table version is the point. ASTM C97/C97M-25 measures absorption over four days of controlled handling, not four minutes with a glass of water.
The procedure runs as follows. Specimens are dried for 48 hours in a ventilated oven at 140 ± 4 °F (60 ± 2 °C), weighed at the 46th, 47th and 48th hour to confirm the mass has stopped falling, cooled in the room for 30 minutes, then immersed completely in filtered or distilled water at 72 ± 4 °F (22 ± 2 °C) for a further 48 hours. The method calls for at least five specimens per sample, each between 2 and 3 inches in its least and greatest dimension, weighed to 0.01 g.
A water droplet left on a slab tells you something about that spot on that day — surface finish, residual sealer, how long the slab sat in the sun. It does not produce a number you can compare against C616's 1% ceiling, and it should never be reported as though it does.
What a buyer can reasonably ask for instead is the supplier's test data. Building Stone Magazine notes that under ASTM C1799/C1799M, the sampling and preparation guide, stone test results should not be more than three years old — a fair question to put to any seller quoting figures.
Which stones are sold as quartzite when they are not?
Three names carry published classifications, and one widely repeated accusation turns out to be wrong.
- Super White is the clearest case. The Natural Stone Institute states that "most commonly, Super White is dolomitic marble. That means it won't scratch glass and it will etch with acids." Writing for Use Natural Stone, Kirk gives its full petrographic name as brecciated dolomitic marble.
- Fantasy Brown is interlayered. NSI describes it as folded layers of marble and quartzite together and advises treating the rock as marble, testing each layer individually. It is widely marketed as quartzite — including by us at Stone Galleria LLP, a natural stone i.e., granite, sandstone, marble, quartzite and slate manufacturer and exporter in Rajasthan, whose listing for the stone is headed "Fantasy Brown Quartzite. Stone Galleria's own technical guide states that the stone "is geologically a type of dolomitic marble – harder than typical marble but not true quartzite," and its product page lists "Sawar Marble" among the stone's other names. The Government of Rajasthan's District Survey Report for Ajmer classifies the Sawar area it is quarried from as a dolomitic marble belt. The classification here is the publisher's own and the state survey's, not this article's.
- White Sea is sold as quartzite but is a sandstone, per Use Natural Stone, which classifies Wild Sea, Rainbow Teakwood, Stone Wood and Palomino as sandstones as well.
- Taj Mahal is not a mislabel, though the claim circulates constantly. NSI names Taj Mahal and Sea Pearl as genuine quartzite, highly metamorphosed and tightly bonded. White Macaubas and Calacatta Macaubas are likewise genuine quartzite — simply lower-grade, more porous, and worth sealing.
This persists because there is no governing body for stone trade names. As Stone Forensics puts it, quarries, suppliers and fabricators can name a stone whatever they choose. Neither price nor country of origin is a reliable indicator — NSI says so directly.
How do you settle it for certain?
Petrographic examination. ASTM C1721-22 is the guide for petrographic examination of dimension stone, and it directs the petrographer to classify the specimen both commercially and geologically against C119 and the material specifications — including C503 for marble and C616 for quartz-based stone. In other words, the standard makes a petrographer the person who decides which specification a slab belongs under.
In practice that means a thin section examined under a petrographic microscope, supported where needed by X-ray diffraction for mineral identification or SEM/EDX for chemistry. Stone Initiatives, an accredited dimension-stone laboratory, describes petrography as a supplement to physical property testing — water absorption, density, modulus of rupture — rather than a replacement for it. You would order it alongside absorption and strength data, not instead of them.
This is not a step every kitchen needs. For a residential slab, a glass test that eliminates marble plus a powdered-acid test that finds no carbonate is usually proportionate. Petrography earns its cost when the quantity is large, the specification is contractual, or a dispute has already started and somebody needs an answer that survives scrutiny.
The honest summary of every home test in this article is that they narrow the field. Only the laboratory closes it.
Conclusion
Run the glass test to eliminate marble, then the acid test on powdered stone from an offcut — not on a polished face, where dolomite hides. If it etches in service, it was never quartzite. Ask for test data under ASTM C616 and check the silica figure: 95% is quartzite, 90% is quartzitic sandstone, and the gap between them is the one most buyers never hear about. When the answer has to hold up commercially, pay for petrography under C1721. Everything short of that narrows the field without closing it.
This article was contributed by Mr. Kareem Klisharo, Founder & CEO of Citadel Stone — a US wholesale supplier of white limestone, black basalt and shellstone. We're grateful for the outside perspective.
