Granite is a widely recognized and durable natural stone, an intrusive igneous rock that forms deep within the Earth from slowly cooling magma. Its composition and formation process give it the physical strength and resistance desirable for architectural and commercial uses. Measuring the hardness of this rock is important for understanding its performance. The standard method used to assess a rock’s resistance to scratching is the Mohs scale of hardness.
Defining the Mohs Scale and Granite’s Hardness Rating
The Mohs scale of mineral hardness is a qualitative ordinal scale ranging from 1 to 10, developed in 1812 by German mineralogist Friedrich Mohs. This scale characterizes the scratch resistance of minerals based on the principle that a harder material will visibly scratch a softer one. The scale is anchored by ten reference minerals, starting with Talc (1) and ending with Diamond (10).
Granite, as a whole rock, typically falls within a range of 6 to 7 on the Mohs scale. This indicates significant resistance to abrasion and scratching. A hardness of 6 is represented by the mineral feldspar on the scale, and a hardness of 7 is represented by quartz.
This numerical rating provides context for granite’s durability compared to common items. For instance, a steel knife blade (around 5.5) will not scratch granite. Furthermore, window glass (5.5 to 6.5) is often softer than the hardest components of granite.
The Role of Mineral Composition in Granite’s Hardness
Granite exhibits a range of 6 to 7 because it is a composite rock, not a single mineral. It is primarily composed of an interlocking matrix of three main minerals: quartz, feldspar, and mica, all of which possess different hardness values. The overall hardness of any given sample of granite is determined by the proportion and size of these component minerals.
Quartz is the hardest of the primary constituents, consistently rating a 7 on the Mohs scale. Feldspar, which is generally the most abundant mineral in granite, rates between 6 and 6.5. The presence of these two hard minerals explains why granite is so scratch-resistant.
The third main component, mica, is significantly softer, with a Mohs rating between 2 and 4. Granite samples with a higher percentage of quartz will trend toward the higher end of the 6 to 7 range. Conversely, those with a greater proportion of feldspar or other softer accessory minerals will fall closer to 6.
Practical Significance of Granite’s Durability
The Mohs hardness rating of 6 to 7 translates directly into granite’s widespread use in demanding applications like countertops, flooring, and exterior cladding. This high scratch resistance means that the stone is harder than most common abrasive materials encountered in a household or commercial setting.
Typical household dust and grit often contain quartz particles, and a material softer than 7 could be gradually scratched by this daily abrasion. Because granite’s hardest component is quartz (7), it is highly resilient against the abrasive effects of common sand and dust, which helps maintain its polished finish over a long period.
The ability to resist scratching from objects like metal utensils, which are generally lower than 6, makes granite an excellent choice for kitchen countertops. This inherent durability minimizes the need for specialized maintenance, ensuring the material remains functional and aesthetically pleasing in high-traffic environments.