Sapphire is a highly valued gemstone, known primarily for its intense color and exceptional durability. This gem is a variety of the mineral corundum, a naturally occurring form of aluminum oxide. Its hardness is a defining physical characteristic, making it a favored material for fine jewelry and demanding industrial applications.
Defining Hardness: The Mohs Scale
The resistance of a mineral to being scratched is quantified using the Mohs scale. This qualitative scale, developed in 1812 by German geologist Friedrich Mohs, ranks minerals from 1 to 10 based on their ability to visibly scratch one another. It is a relative measurement, meaning a mineral with a higher number can scratch any mineral with a lower number.
The scale begins with talc at 1, the softest mineral, and culminates with diamond at 10, the hardest known natural substance. A higher number indicates greater resistance to surface abrasion. The scale is ordinal rather than linear, meaning the difference in absolute hardness between consecutive numbers varies significantly, such as the large jump between 9 and 10.
Sapphire’s Measured Hardness and Corundum Structure
Sapphire is rated as a 9 on the Mohs scale. This places it directly below diamond and confirms its status as one of the hardest natural minerals. Both sapphire and its red counterpart, ruby, are gem varieties of corundum, which is chemically crystalline aluminum oxide.
The scratch resistance of corundum is a direct result of its atomic structure. Aluminum and oxygen atoms are tightly packed into a dense, hexagonal crystal lattice. This compact arrangement creates exceptionally strong atomic bonds throughout the structure, and the energy required to break these bonds determines the high Mohs rating of 9.
Practical Durability and Comparison to Other Materials
The Mohs 9 rating translates into superior real-world performance, especially when compared to common abrasive materials. Household dust and airborne grit often contain quartz, which measures 7 on the Mohs scale. Since sapphire is a 9, it cannot be scratched by quartz dust, making it an excellent choice for items that endure constant exposure, such as wristwatch crystals and daily-wear jewelry.
Sapphire’s hardness also surpasses that of many other materials used in jewelry, including common glass, which typically sits below 6. Its durability is further enhanced by its excellent toughness, which is the ability to resist breaking or chipping from impact.
Unlike some materials, sapphire has no distinct planes of weakness, called cleavage, meaning it resists splitting when struck sharply. Though diamond is harder, sapphire remains a highly durable and stable choice for rings and other pieces subject to daily physical stress.