Sapphire is the single-crystal form of aluminium oxide (Al₂O₃) and a member of the corundum mineral family. Known for its outstanding hardness, optical transparency, and thermal resilience, sapphire features a hexagonal crystal structure that supports a wide range of demanding applications. It offers excellent transmission from the ultraviolet through the visible and into the infrared spectrum, making it a preferred material for optical windows, lenses, and prisms. Thanks to its extreme scratch resistance, sapphire is also used in durable consumer components such as smartphone screens, watch crystals, and protective covers. In aerospace and defence, it serves in high-temperature environments, optical sensor windows, and bullet-resistant transparencies.