sapphires

Sapphires are gem-quality varieties of the mineral corundum, aluminum oxide (Al2O3). In gemology, sapphire refers to all colors of corundum except red, while red corundum is called ruby. The most familiar color is blue, produced by trace amounts of iron and titanium, but sapphires occur in nearly every color, including pink, yellow, orange, green, purple, and colorless. Fancy sapphires describe the non-blue colors, and some stones show asterism or chatoyancy due to inclusions.

Properties and characteristics: Corundum has a hardness of 9 on the Mohs scale, making sapphires among the hardest natural gemstones and highly durable for jewelry. They belong to the hexagonal (trigonal) crystal system and often display pleochroism, with color that may vary when viewed from different directions. Sapphires can be transparent to opaque and possess a high refractive index and good brilliance when well cut.

Origin and sources: Sapphires form in a variety of geological settings, including metamorphic and igneous rocks. Important sapphire producing regions include Sri Lanka (Ceylon), Myanmar (Burma), Madagascar, Australia, Thailand, and several African countries. Blue sapphires are the most widely known, though valuable pink, purple, and orange stones also occur.

Treatments and synthetic varieties: Many sapphires are heated to improve color and clarity; other enhancement methods (diffusion treatments, fracture filling, irradiation) are used on some stones. Synthetic sapphires produced by methods such as Verneuil, Czochralski, and flux growth are chemically identical to natural sapphires and are widely used in jewelry and industry.

Value and care: Value depends on color quality, clarity, cut, and carat weight, as well as origin and treatment status. Common care tips include gentle cleaning and avoiding harsh ultrasonic cleaners on stones with inclusions.

Notable sapphires: Logan Sapphire, Star of India, and the Hope Sapphire are among the best-known large sapphires.