Sapphire (antique greek: hyacinthos) refers to gem varieties of the mineral corundum, an aluminium oxide (Al₂O₃), when it is a color other than red (ruby) or pinkish-orange (padparadscha). Sapphire can be found naturally, or manufactured in large crystal boules. Because of its remarkable hardness, sapphire is used in many applications, including infrared optical components, watch crystals, high-durability windows, and wafers for the deposition of semiconductors, such as GaN nanorods and blue LEDs.

The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Sapphire includes any gemstone-quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as the ruby, and the pinkish-orange variety known as padparadscha.

Natural sapphire

Sapphire is one of the two gem varieties of the species corundum. Although blue is the best known hue, the gem occurs in virtually every spectral hue excepting red; red corundum is ruby. Sapphire may also be colorless, and it also occurs in the non-spectral shades gray and black.

The cost of sapphire gems varies depending on their color, clarity, size, cut, and overall quality. As of 2000, the cost of 1 carat (0.2 g) of a typical uncut, gem quality sapphire was about $1,600 USD. Significant sapphire deposits are found in Eastern Australia, Southeast Asia, and Sri Lanka. Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.

Blue sapphire

Various shades of blue and light result from titanium and iron substitutions in the aluminum oxide crystal lattice. Some stones are not well saturated and show tones of gray. It is common practice to bake natural sapphires to improve or enhance color. This is usually done by heating the sapphires to temperatures of up to 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. On magnification, the silk due to included rutile needles are often visible. If the needles are unbroken, then the stone was not heated; if the silk is not visible then the stone was heated adequately. If the silk is partially broken, then a process known as low tube heat may have been used. Low tube heat is the process whereby the rough stone is heated to 1300 °C over charcoal for 20 to 30 minutes. This removes gray or brown in the stone and improves color saturation.

Fancy color sapphire

Purple sapphires are lower in price than blue ones. These stones contain the trace element vanadium and come in a variety of shades. Yellow and green sapphires have traces of iron that gives them their color. Pink sapphires have a trace of the element chromium and the deeper the color pink the higher their monetary value as long as the color is going toward the red of rubies. Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Salmon-color padparadscha sapphires often fetch higher prices than many of even the finest blue sapphires. The word 'padparadscha' is Sinhalese for 'lotus flower'. Recently many sapphires of this color have appeared on the market as a result of a new treatment method called "lattice diffusion".Fact: date=April 2007