Sapphire

Sapphire is a precious gemstone variety of the mineral corundum, aluminum oxide (Al2O3). It is most renowned for its blue color, but sapphires occur in nearly every color, including yellow, orange, pink, green, purple, and colorless. The blue shade is typically produced by trace amounts of iron and titanium; the red variety of corundum is called ruby.

The name sapphire derives from ancient languages meaning blue stone. In gemology, the term "fancy sapphires" describes non-blue stones, while padparadscha refers to pink‑orange sapphires. Blue sapphires from Kashmir, Burma, and Sri Lanka are historically highly prized, but significant stones have also come from Madagascar, Australia, Thailand, and the United States.

Sapphires crystallize in the trigonal (rhombohedral) system as part of the corundum family. They form in high-temperature environments in igneous and metamorphic rocks and are frequently found with other alumina minerals. They are mined from primary deposits and alluvial placers.

Key properties include a Mohs hardness of 9, making sapphire one of the hardest natural substances after diamond. They exhibit relatively high refractive indices and strong brilliance, contributing to their value as gems. Sapphire is chemically stable, though many blue stones are treated to enhance color and clarity.

In addition to natural gems, synthetic sapphires are produced by methods such as the Verneuil and Czochralski processes for jewelry and industrial use. Industrial-grade sapphire is used for durable watch crystals, smartphone screens, LED substrates, and laboratory equipment. Star sapphires show asterism caused by rutile inclusions under direct light.