Due to its widespread use in construction and countertops, many people assume granite is the hardest substance on Earth. Granite is an intrusive igneous rock, formed deep beneath the Earth’s surface from the slow cooling and crystallization of magma. This formation process yields a rock with considerable strength and durability, making it a popular choice for high-traffic areas. While granite is certainly a robust material, it is a common misconception that it holds the title for ultimate mineral hardness.
How Geologists Measure Hardness
The scientific community uses a specific standard to evaluate a material’s resistance to scratching, a property known as mineral hardness. This measurement is most often determined using the Mohs scale of mineral hardness, a non-linear ranking system. The scale assesses how easily one material can be visibly scratched by another, providing a relative comparison between minerals.
The scale ranges from 1, represented by the soft mineral talc, up to 10, which is the maximum hardness achieved by diamond. A material’s Mohs number is assigned by finding the hardest material that the specimen cannot scratch, and the softest material that can scratch the specimen. For example, if a specimen is scratched by a material with a rating of 7 but not by a rating of 6, its hardness is considered to be between 6 and 7. This comparative method provides a consistent way for geologists to classify the resistance of minerals to surface deformation.
Granite’s Specific Hardness and Composition
Granite is not a single mineral but rather an aggregate rock composed of several distinct minerals, which determines its overall hardness rating. The primary components are quartz, various types of feldspar, and mica, with proportions varying based on the rock’s origin. Because it is a mixture, its scratch resistance is composite and not uniform across its entire surface.
The hardness of granite is largely influenced by the presence of quartz, which is one of the harder minerals in the mixture and typically scores a 7 on the Mohs scale. Feldspar, another major component, registers slightly lower, generally falling between 6 and 6.5. Mica is significantly softer, rating only between 2 and 4.
When considering the entire rock structure, the composite material generally exhibits a Mohs hardness rating that falls between 6 and 7. This range explains why granite is not easily scratched by common items like steel knife blades, which usually have a hardness lower than 6. However, this rating also means that any material scoring a 7 or higher can easily leave a permanent mark on the granite’s surface.
The rock’s popularity in construction is based not only on its scratch resistance but also on its toughness. Hardness refers specifically to resistance to surface scratching, while toughness is the ability of a material to absorb energy before fracturing or breaking. Granite’s crystalline structure makes it highly resistant to breaking under impact, a property distinct from its Mohs hardness rating.
Materials That Are Harder Than Granite
Since granite typically peaks at a Mohs rating of 7, any material scoring higher can scratch and damage it. One step above granite’s hardest components is Topaz, assigned a Mohs rating of 8. Topaz is a silicate mineral that can easily abrade the surface of granite.
Significantly harder than Topaz is Corundum, which is rated 9 on the Mohs scale. Corundum is the crystalline form of aluminum oxide, and its pure gemstone varieties are known as ruby and sapphire. Materials containing corundum are frequently used as industrial abrasives and in high-performance cutting applications because they can easily slice or grind through granite surfaces.
At the very top of the scale, diamond achieves the maximum rating of 10, making it the hardest known naturally occurring material. Diamond’s superior hardness means it can scratch every other material, including corundum and granite. This characteristic is why diamond is used extensively in drill bits, saw blades, and grinding wheels designed to cut through rock and other extremely hard substances.
Engineered materials often used in construction also compete with granite in hardness. Engineered quartz-based materials, made primarily of ground quartz crystals bound by resin, consistently possess a hardness around 7, making them comparably scratch-resistant to granite’s hardest components. Natural quartzite, formed from sandstone, is also composed mostly of quartz and registers a hardness of 7 to 8, making it frequently harder than granite.