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Birefringent Crystal
Sapphire is an anisotropic birefringent crystal, which means that it has two optical axes at right angles to each other, said the MicroscopyU.com website. Light beams entering the crystal parallel to an optical axis will pass straight through, but those entering at an angle to both optical axes will split into two beams traveling along the two optical axes at different speeds, emerging in slightly different places and vibrating in planes that are mutually perpendicular. This property makes sapphire excellent for industrial polarized light filters. In sapphire gemstones, this property means the shade and intensity of the color will change with the angle of view.
Industrial Sapphire
Humans put aluminum oxide under high heat and great pressure to produce man-made sapphire used in industrial and commercial applications requiring transparency, scratch resistance, heat resistance, dimensional stability and resistance to compression and corrosive chemicals, according to London-based optical materials fabricator Roditi International. Sapphire can be fabricated into disks, rods, tubes, ingots, pins, plates and covers. Applications include windows and lenses, laboratory optics, watch crystals, electronic wafers and airport runway light covers.
Sapphire Gemstones
Natural sapphires formed eons ago deep under the ground, colored by trace amounts of other elements. Sapphires are gemstones of the corundum group, according to the International Colored Gemstone Association website. Most natural sapphires are a shade of blue from iron and titanium traces, but the gem also occurs in yellow, purple, pink and green shades, as well as colorless stones. Sapphires must be cut and faceted to bring out their color. Most are mined in Thailand or Australia. But natural sapphires also occur in India, Burma, Sri Lanka, Vietnam, Brazil, Madagascar and Tanzania.