Sapphire
Sapphire is a gemstone variety of the mineral corundum, the crystalline form of aluminium oxide (α-Al2O3). Pure corundum is colorless, while trace elements in its crystal structure produce a wide range of colors. Blue is the best-known, but sapphires also occur in yellow, green, purple, pink, orange, gray, black, and colorless forms. Red gem-quality corundum is classified as ruby rather than sapphire, although the boundary between ruby and pink sapphire is not universally defined and varies among gemological and trade conventions. Sapphire has a Mohs hardness of 9, making it highly resistant to scratching and abrasion.
Natural sapphires occur in a range of metamorphic and magmatic geological settings and are also recovered from secondary alluvial deposits. Their color and appearance reflect trace-element chemistry, crystal growth, and microscopic inclusions. Some sapphires exhibit distinctive optical phenomena, notably asterism in star sapphires and color change under different illumination. As gemstones, sapphires are evaluated principally by their color, together with clarity, cut, size, treatment history, and, for some stones, geographic origin.
Natural sapphires are commonly heat-treated to modify their color and clarity, while diffusion treatments can also alter their coloration. The detection and disclosure of such treatments are important aspects of modern gemology. Synthetic sapphire is laboratory-grown single-crystal corundum (α-Al2O3). Commercial production of synthetic corundum became practical in the early 20th century with the Verneuil process, and sapphire is now grown by several crystal-growth methods for both gemological and industrial uses.
Beyond its use as a gemstone, high-purity synthetic sapphire is an important optical and engineering material. Its hardness, abrasion resistance, broad optical transparency, high-temperature stability, and electrical insulating properties enable its use in watch crystals, protective and optical windows, scientific instruments, and semiconductor substrates. Sapphire wafers are used in silicon on sapphire electronics and as substrates for gallium nitride devices, including blue LEDs, while doped sapphire crystals are used as laser gain media, notably in titanium-sapphire lasers.
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