Granite registers between about 6 and 7 on the Mohs hardness scale, which puts it among the harder natural stones used in construction and design. That range rather than a single number reflects an important reality: granite is a rock, not a mineral, and its hardness depends on the proportions of different minerals locked together inside it. The answer is straightforward, but understanding what that 6-to-7 range actually means in practice requires a closer look at what granite is made of and how hardness testing works on something that is not uniform throughout.
Why Granite Gets a Range Instead of a Single Number
The Mohs scale was designed for individual minerals, not for composite rocks. Each of the ten reference minerals on the scale is a single chemical substance with a consistent crystal structure: talc at 1, gypsum at 2, calcite at 3, and so on up to diamond at 10. When you scratch one mineral against another, the result is predictable and repeatable because both materials have uniform compositions. Granite, however, is a coarse-grained igneous rock formed from cooled magma, and it contains several different minerals locked together in an interlocking crystalline texture. The hardness you measure at any given spot on a granite surface depends on which mineral your testing point happens to land on.
The three main mineral players in most granites are quartz, feldspar, and mica. Quartz sits at 7 on the Mohs scale and is typically the hardest component. Feldspar, which comes in two main varieties (orthoclase and plagioclase), sits at 6. Mica is much softer, around 2.5 to 3. When someone says granite is “6 to 7 on the Mohs scale,” they are really averaging across these minerals while leaning toward the harder constituents that dominate the rock’s surface behavior. The quartz-rich spots will resist scratching at the level of Mohs 7, while the feldspar-dominated areas will give way to anything harder than Mohs 6.
What the Minerals Actually Contribute
A typical granite is roughly 20 to 60 percent feldspar and 20 to 40 percent quartz, with smaller amounts of mica and accessory minerals like hornblende or magnetite. The exact recipe varies enormously depending on where the rock formed and how quickly its magma cooled. That variation in mineral proportions is the primary reason one slab of granite can feel meaningfully harder or softer than another.
Laboratory measurements bear this out. When researchers measure the indentation hardness of granite’s main minerals at room temperature, quartz comes in at around 12.89 GPa, plagioclase feldspar at about 9.49 GPa, and orthoclase feldspar at roughly 8.96 GPa.1Tectonophysics. Low-temperature plastic rheology of granitic feldspar and quartz Those numbers use a more precise scale than Mohs, but the pattern lines up: quartz is substantially harder than either type of feldspar. A granite sample with a high percentage of quartz will behave as a harder material overall, while a feldspar-rich granite will be somewhat easier to scratch, cut, and polish.
Mica is the wild card. It is soft enough that a fingernail can scratch it, and flakes of biotite or muscovite scattered through granite create weak points. In most commercial granites these soft inclusions are a small percentage of the total volume, so they do not dramatically lower the rock’s surface hardness. But in granites with visible, abundant mica flakes, those soft zones become more noticeable and can affect wear resistance over time.
The Mohs Scale Is Cruder Than You Might Think
People often treat the Mohs scale as though it measures hardness in even steps, the way a ruler measures length. It does not. The gaps between adjacent Mohs numbers are wildly uneven. The jump from corundum (9) to diamond (10) represents a much larger increase in absolute hardness than the jump from quartz (7) to topaz (8). Research into the measured microhardness, toughness, and elastic modulus of the Mohs reference minerals confirms that none of these properties increases consistently or linearly across the scale.2American Mineralogist. Microhardness, toughness, and modulus of Mohs scale minerals In other words, the Mohs number tells you which material will scratch which, but it does not tell you how much harder one material is than another in any absolute sense.
For granite, this matters because the difference between a Mohs 6 and a Mohs 7 mineral is not as dramatic in practical terms as the scale might suggest. Both quartz and feldspar are hard enough to resist scratching by a steel knife blade (about Mohs 5.5), and both are hard enough to scratch ordinary glass (about Mohs 5.5). The real-world distinction between the two shows up mainly in situations involving sustained abrasion or contact with materials right at the boundary, like certain ceramics or hardened steel tools.
How Granite’s Texture and Grain Size Affect Its Toughness
Hardness is not the whole story when it comes to how granite performs in everyday use. Two slabs of granite with identical mineral proportions can behave differently if their internal textures differ. Research on the petrographic features of granites shows that grain size, the distribution of those grain sizes, and the way minerals are packed together all exert significant control over mechanical properties like compressive strength and resistance to fracture.3Engineering Geology. Petrographic features as an effective indicator for the variation in strength of granites A granite with fine, tightly interlocked grains tends to be tougher than one with large, loosely arranged crystals, even if both contain the same minerals in the same proportions.
Grain size heterogeneity, meaning how much variation exists among crystal sizes in a single sample, also plays a role. When mineral grains are a mix of very large and very small crystals, the boundaries between them become stress concentrators. This effect is especially pronounced when temperature changes come into play, because different-sized grains expand and contract at slightly different rates. Studies have found that granite with a higher degree of grain size variation tends to develop microcracks at lower temperatures than granite with more uniform grains.4International Journal of Rock Mechanics and Mining Sciences. Grain size heterogeneity controls strengthening to weakening of granite over high-temperature treatment In a kitchen countertop, this effect is marginal because the temperature swings are small. But in outdoor applications, repeated freeze-thaw cycles or prolonged sun exposure can gradually exploit these grain boundaries and weaken granite surfaces over years.
Recrystallization along grain boundaries, which happens naturally in granites that have been subjected to moderate heat and pressure over geologic time, actually strengthens the rock by tightening those junctions.3Engineering Geology. Petrographic features as an effective indicator for the variation in strength of granites So the geologic history of a particular granite deposit shapes how it will perform in construction, not just its mineral recipe.
What Can and Cannot Scratch Granite
If you are choosing granite for a countertop or floor and want to know what everyday materials might damage it, the Mohs scale gives you a rough guide. Anything softer than Mohs 6 should not be able to scratch the surface. That includes fingernails (Mohs 2.5), copper coins (Mohs 3.5), steel knives (Mohs 5 to 5.5), and most glass (Mohs 5.5). You can chop vegetables directly on a granite countertop without scratching it, though you will quickly dull the knife.
Materials that can scratch granite include anything at Mohs 7 or above. Quartz itself is the most common culprit: sand and dust often contain tiny quartz particles, which is why dragging a gritty object across granite can leave fine scratches. Topaz, sapphire, and diamond all rank above quartz and will scratch granite easily. The diamond blades used in stone fabrication shops exist precisely because diamond is the only common material hard enough to cut through quartz-rich granite efficiently.5IOP Conference Series: Materials Science and Engineering. Flexible Tools with Diamond Blades for Polishing Granite Surfaces
Ceramic dishes and some types of stoneware can be tricky. Many ceramics contain alumina or silica compounds with hardness values near or above 7, which means dragging the bottom of a rough ceramic mug across granite can leave a mark. Those streaks are sometimes the ceramic material depositing itself onto the granite rather than the granite being scratched, but genuine scratches are possible too. If you notice white or gray lines on a dark granite surface, try wiping them with a damp cloth first; if they come off, the granite is fine.
Why Granite Wears Differently Than Hardness Alone Would Predict
Scratch hardness and wear resistance are related but not identical. A material can be very hard on the Mohs scale yet wear unevenly when subjected to prolonged abrasion. Granite is a good example: its abrasion behavior does not follow the straightforward linear pattern you might expect from its hardness rating. Research on stone flooring materials found that granites show a non-linear wear pattern under sustained contact, meaning the rate of material loss does not scale predictably with the applied load.6Construction and Building Materials. Abrasion wear characterization of some selected stone flooring materials with respect to contact load
This non-linearity happens because granite is a composite. As an abrasive force works across the surface, it encounters alternating zones of hard quartz, moderately hard feldspar, and soft mica. The softer minerals wear away faster, eventually undermining adjacent harder grains and causing them to pop out rather than wear down smoothly. The result is that granite floors in high-traffic areas develop a subtly pitted texture over decades, with the quartz grains standing slightly proud of the surrounding feldspar. This is why polished granite lobby floors in old buildings often feel rougher than they did originally; the polish is not just worn away but selectively removed from the softer mineral zones first.
This composite wear behavior also explains why the micro-hardness profile of a granite slab correlates well with how efficiently it can be sawn and ground in a fabrication shop. The distribution of hard and soft zones across the surface predicts sawing efficiency and tool wear better than a single bulk hardness number does.7Journal of Materials Processing Technology. Parameterization of micro-hardness distribution in granite related to abrasive machining performance For polishing, however, the final visual result depends more on the optical properties of the minerals than on their hardness, which is why some granites take a brilliant mirror polish while others with similar hardness look dull.
How “Granite” in a Stone Yard May Not Be Granite at All
One complication worth knowing about is that the stone industry uses the word “granite” much more loosely than geologists do. In a stone yard or kitchen showroom, “granite” often refers to any hard, coarse-grained igneous or metamorphic rock with a speckled appearance. This includes true granite but also gabbro, anorthosite, gneiss, and sometimes even dense varieties of marble or quartzite that have been commercially labeled as granite for marketing purposes.
These commercial “granites” have widely varying hardness. True granite lands in the 6-to-7 range because of its quartz and feldspar content. Gabbro, which is rich in pyroxene and plagioclase but typically low in quartz, might sit closer to 6. Quartzite, which is almost entirely quartz, consistently hits 7 and resists scratching better than any true granite. Marble, which is primarily calcite at Mohs 3, is far softer and will scratch if a steel knife touches it. If you are counting on the hardness of granite to protect your countertops, it is worth asking what the stone actually is geologically, not just what the label says.
A simple home test can help sort this out. Try scratching an inconspicuous spot with a steel nail or the blade of a pocket knife. If the stone scratches easily, it is probably not true granite or quartzite. If the knife slides across without leaving a mark, you are likely dealing with a stone that sits at Mohs 6 or above. This is not definitive, but it can catch the most common mislabeling issue, which is softer metamorphic stones being sold under the granite umbrella.
Granite Versus Engineered Quartz and Other Countertop Materials
In the countertop market, granite competes with engineered quartz (brands like Silestone and Caesarstone), quartzite, marble, soapstone, and solid-surface materials like Corian. Hardness is one of the main selling points for granite, so it helps to know where it actually sits relative to these alternatives.
Engineered quartz slabs are made from roughly 90 to 94 percent ground natural quartz bound with polymer resins. Because the quartz content is so high and uniformly distributed, these surfaces behave like Mohs 7 across their entire area, without the soft mica or feldspar zones that exist in natural granite. The trade-off is that the resin binder is heat-sensitive and can discolor or scorch under very hot pans, something that rarely happens with natural granite. Quartzite, the natural metamorphic rock, is also almost pure quartz and matches engineered quartz in scratch resistance, but it is rarer and more expensive.
Marble, at Mohs 3, is significantly softer than granite and will scratch, etch, and stain from acidic foods. Soapstone sits around Mohs 1 to 2 when it is talc-rich, though harder varieties reach about 5. Solid-surface acrylics like Corian are roughly Mohs 2 to 3. Compared to all of these, granite’s 6-to-7 rating represents a genuine practical advantage for scratch resistance, though it is not the hardest option available.
Where granite has an edge over engineered quartz is heat resistance. Because granite is entirely mineral with no polymer component, you can set a hot pan on it without worrying about scorch marks or melting. For people who cook frequently and do not want to fuss with trivets, this is a meaningful distinction that hardness numbers alone do not capture. Durability in daily use comes from a combination of scratch resistance, heat tolerance, stain resistance, and impact strength, and no single number on any scale tells the whole story.
How Granite Holds Up Outdoors
Granite’s hardness makes it one of the most weather-resistant natural building stones, which is why it has been used for centuries in monuments, bridges, and building facades. Its quartz and feldspar grains are largely resistant to chemical weathering by rainwater, and its interlocking crystal structure makes it difficult for water to penetrate deeply. The result is a stone that can last centuries in exposed conditions with relatively little deterioration compared to limestone or sandstone.
That said, granite is not indestructible outdoors. Freeze-thaw cycles can exploit microcracks between grains, slowly widening them over many years. As noted earlier, granites with heterogeneous grain sizes are more vulnerable to this because their grain boundaries are already under some internal stress from differential thermal expansion.4International Journal of Rock Mechanics and Mining Sciences. Grain size heterogeneity controls strengthening to weakening of granite over high-temperature treatment In very cold climates, granite steps and curbstones develop a roughened surface over decades as individual grains loosen and pop out. This is a slow process, but it is why ancient granite structures show rounded, pitted surfaces rather than the smooth polish they may have started with.
Biological weathering also affects outdoor granite. Lichens and mosses colonize the surface and produce weak organic acids that slowly dissolve feldspar grains. This is almost invisible on a human timescale but becomes obvious on structures that are hundreds of years old. The quartz grains, being chemically more resistant, tend to remain intact while the feldspar weathers away around them, giving very old granite surfaces a sandpaper-like texture. None of this changes the mineral hardness itself, but it is a reminder that hardness is not the only factor determining how long a stone surface stays smooth and intact.