Sapphire is a variety of the mineral corundum, characterized by its composition of aluminum oxide ($\text{Al}_2\text{O}_3$). Chemically, its appearance and distinct coloration are due to trace amounts of elements, most commonly iron and titanium, which act as chromophores, giving rise to blues, but also hues of yellow, pink, and red. Crystallographically, the mineral possesses a hexagonal crystal structure, making it durable and relatively hard, ranking high on the Mohs scale of mineral hardness. Its physical properties, such as its vitreous luster and pleochroism, make it subject to intensive study within the fields of crystallography and optical physics.