Corundum is a naturally occurring mineral composed primarily of aluminum oxide (\(\text{Al}_2\text{O}_3\)), which forms under intense heat and pressure deep within the Earth. While pure corundum is colorless, the presence of trace impurities creates the world’s most durable gemstones. These gem varieties include ruby, colored red by chromium, and sapphire, which encompasses all other colors due to elements like iron and titanium. This exceptional physical durability allows corundum to persist in geologic environments and supports its widespread use in jewelry and heavy industry.
Quantifying Corundum’s Resistance
The scratch resistance of corundum is measured precisely on the Mohs scale of mineral hardness, where it is assigned a value of 9. The Mohs scale is a qualitative system based on the ability of one material to visibly scratch a softer material. This relative ranking means that any mineral with a lower Mohs number, such as quartz at 7 or topaz at 8, will be scratched by corundum.
Corundum holds the rank of the second hardest natural mineral, surpassed only by diamond at a Mohs value of 10. Although only one point separates them, the Mohs system is not linear, and the difference in absolute hardness is substantial. Diamond is estimated to be nearly four times harder than corundum, demonstrating the immense leap in strength at the very top of the scale.
The Atomic Structure Responsible for Strength
The profound hardness of corundum stems from its precise internal atomic arrangement, a dense crystalline structure of aluminum oxide (\(\text{Al}_2\text{O}_3\)). Corundum forms in a trigonal crystal system with a hexagonal close-packed arrangement of oxygen ions. Within this tightly packed lattice, the smaller aluminum ions occupy two-thirds of the available octahedral spaces. The bonding between the aluminum and oxygen ions is a mix of ionic and covalent character, creating strong, short bonds that are incredibly difficult to break. The lack of easy cleavage planes in the crystal further contributes to its mechanical toughness.
Industrial and Technological Uses
Corundum’s extreme hardness makes it a highly valued material for a variety of industrial and technological applications. The most common use leverages its abrasive properties, as the mineral can effectively cut and shape nearly all other materials. Natural corundum that is not gem-quality is often crushed and processed to create sandpaper, grinding wheels, and polishing compounds used in machining metals, wood, and glass.
Synthetic and High-Tech Applications
Synthetic corundum, manufactured in controlled environments, is indispensable in high-tech fields requiring extreme durability and optical clarity. Its superior scratch resistance leads to its use in specialized optics, such as scratch-resistant watch crystals and the protective windows for electronic instruments. Corundum is also employed in mechanical components, like industrial bearings and precision jewel bearings in watches, where its hardness ensures minimal wear over long periods of operation. Beyond its mechanical applications, corundum’s high melting point of over 2,000° C allows it to be used in refractories for lining furnaces and kilns.