Sapphire, known for its brilliant color and hardness, is a gem-quality variety of the mineral corundum. Corundum is composed of aluminum oxide (\(\text{Al}_2\text{O}_3\)) and serves as the index mineral for a hardness of nine on the Mohs scale. Understanding how this mineral responds to stress is fundamental to gemology and mineral identification. The way a mineral breaks provides significant clues about its internal atomic structure, determining its value and application. This analysis focuses on the specific breakage mechanism of sapphire—whether it exhibits cleavage or fracture.
Defining Mineral Breakage: Cleavage and Fracture
Minerals break apart in two distinct ways, reflecting the strength and arrangement of their internal atomic bonds. Cleavage describes the tendency of a crystal to split smoothly along flat, predictable planes of weakness. These planes exist where the atomic bonds are significantly weaker in one direction compared to others. The result is a clean, flat surface that is parallel to a potential crystal face.
In contrast, fracture occurs when a mineral breaks irregularly and randomly, without following any specific crystallographic direction. This type of breakage happens when the internal atomic bond strength is roughly equal in all directions. A common type of irregular break is the conchoidal fracture, which produces smooth, curved, shell-like surfaces, similar to broken glass.
The Breakage Pattern of Sapphire (Corundum)
The breakage pattern of sapphire is definitively categorized as fracture, not cleavage. True cleavage, which involves a smooth, repeatable separation along a weak plane, is absent in corundum. When sapphire is broken, the resulting surfaces are typically uneven or exhibit a conchoidal fracture.
Corundum can sometimes display a feature called parting, which can be confused with cleavage. Parting is a pseudo-cleavage that occurs along crystallographically defined planes. However, it is caused by mechanical stress or twinning planes rather than inherent bond weakness. Unlike true cleavage, parting does not occur in every specimen and cannot be repeated indefinitely, confirming that fracture remains the dominant breakage pattern.
Crystalline Structure and the Absence of Cleavage
The lack of cleavage in corundum is a direct consequence of its highly stable and uniform crystalline structure. Sapphire is composed of aluminum oxide (\(\text{Al}_2\text{O}_3\)) arranged in a trigonal (hexagonal) crystal system. Within this structure, the large oxygen ions form a dense, close-packed lattice, with the smaller aluminum ions occupying two-thirds of the available octahedral sites.
The bonds between the aluminum (\(\text{Al}_2\text{O}_3\)) and oxygen ions are extremely strong, exhibiting a mixture of ionic and covalent character. These strong bonds are distributed with near-uniformity throughout the three-dimensional lattice. Because the strength of the atomic attraction is consistently high in all crystallographic directions, there is no preferred path of least resistance for a crack to follow. When sufficient force is applied, the break must sever bonds of equal strength, resulting in the observed conchoidal fracture.
Impact on Cutting and Durability
The fracture behavior of sapphire has significant practical implications, contributing to its reputation as an exceptionally durable gemstone. Sapphire’s combination of extreme hardness (Mohs 9) and the absence of true cleavage makes it resistant to both scratching and breaking. Unlike minerals with perfect cleavage, such as diamond or topaz, a blow to a sapphire will not easily cause it to split cleanly along a defined internal plane.
For gem cutters, this means they do not have to worry about accidentally cleaving the stone when shaping or mounting it. The material’s ability to fracture randomly, rather than cleave, is what makes it tough, meaning it resists the propagation of cracks. The strength derived from this uniform bonding is why sapphire is widely used in jewelry and in industrial applications requiring high mechanical strength, such as scratch-resistant watch crystals.