Granite is a durable, naturally occurring stone, commonly used in construction and countertops due to its resistance to scratching and wear. In materials science, “hardness” refers to a material’s resistance to permanent shape change when a force is applied. While granite is hard in common usage, many other materials, both natural and synthetic, are significantly harder.
Defining Material Hardness
The most common way to classify mineral hardness is the Mohs scale, which ranks materials from 1 to 10 based on their ability to scratch one another. This test measures scratch resistance; a higher-rated material can visibly mark any material with a lower rating. Granite, a mix of minerals, typically ranks between 6 and 7, resisting scratching from common objects like a steel knife (around 5.5).
The Mohs scale is relative, meaning the jump in hardness between a 9 and a 10 is much greater than between a 1 and a 2. Scientists also use quantitative methods, such as the Vickers and Brinell tests, which measure a material’s resistance to permanent indentation. These tests apply a precise force, yielding a numerical pressure value for a more exact measure of overall strength.
Materials That Exceed Granite
Materials that surpass granite’s Mohs rating of 6-7 are characterized by strong, short atomic bonds, making them difficult to break or scratch. Natural minerals begin this list: Topaz scores an 8. Corundum, which includes Ruby and Sapphire, registers a 9, capable of scratching any mineral component found in granite.
At the top of the natural hardness scale is Diamond, assigned a 10 and exponentially harder than the material ranked just below it. Beyond natural minerals, synthetic materials are engineered for extreme hardness and durability. Silicon Carbide, a ceramic compound, is a widely used industrial abrasive that registers a Mohs hardness of 9 to 9.5.
Boron Carbide, a tough ceramic nicknamed “black diamond,” falls in the range of 9.3 to 10 Mohs, making it the third hardest material known, just behind diamond and cubic boron nitride. Cubic Boron Nitride (c-BN) is another synthetic material with hardness approaching that of diamond, but offering superior chemical stability when machining iron-based metals. These materials utilize covalent bonding to achieve extreme resistance to wear.
Applications of Ultra-Hard Materials
Materials significantly harder than granite are essential for modern manufacturing and technology. These ultra-hard substances are primarily used to cut, grind, or shape other hard surfaces without degrading themselves. Diamond and cubic boron nitride are fabricated into specialized cutting tools and drill bits used in rock drilling, mining, and machining hardened steel and aerospace alloys.
Silicon Carbide and Boron Carbide are commonly used as abrasive grit in sandpaper, grinding wheels, and lapping compounds. Due to their wear resistance, they are also used as protective coatings on mechanical parts like pump seals and sandblasting nozzles. The unique thermal and electrical properties of synthetic diamond and silicon carbide are leveraged in specialized electronics, such as heat spreaders and semiconductor components.