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sapphire information
Natural sapphire
Although
blue is considered the normal color for sapphire, it is found in the
full range of spectral colors as well as brown, colorless, grey and black.
Any sapphire other than blue or fully saturated red (ruby)
is considered a fancy color sapphire.
Blue sapphire
Various shades of blue [dark and light] result from titanium and iron substitutions in the aluminium oxide crystal lattice. Some stones are not well saturated and show tones of gray. It is common to bake natural sapphires to improve 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 7 days or more. On magnification the silk due to included rutile needles are 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 was used. Low tube heat is the process where the rough stone is heated to 1300 °C for 20 to 30 minutes over charcoal. This takes out any gray or brown in the stone and improves color saturation.
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Fancy color sapphire
Purple sapphires are lower in price than blue ones. These stones contain the
trace element vanadium and
come in a wide variety of shades. Yellow and green sapphires have traces of
iron which gives them their color. Pink sapphires have a trace element of chromium and
the deeper the color pink the higher the value as long as the color is going
toward 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
are orangy-pink, pinkish-orange, or pink-orange in color and often fetch higher
prices than many of 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 "bulk diffusion"
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Color change sapphire
Color shift sapphires are blue in outdoor light and purple in indoor light. Color changes may also be pink in daylight to greenish in fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to blue purple. White sapphires usually come out of the ground as light grey or brown and are then heated to make them clear. However in very rare circumstances they will be found in a clear state.
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Star sapphire
A star sapphire is a type of sapphire that exhibits a star-like
phenomenon known as asterism.
Star sapphires contain intersecting needle-like inclusions (often the mineral rutile)
that cause the appearance of a six-rayed 'star'-shaped pattern when viewed
with a single overhead light source.
The value of a star sapphire depends not only on the carat weight
of the stone but also the body color, visibility, and intensity of the star.
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Treatments
Some sapphires are heat-treated or otherwise enhanced to improve their appearance and color, though some people object to such practices and prefer natural untreated stones. Heat treatments tend to improve the sapphire's color and clarity, and it is unusual to find un-heated stones for sale. Diffusion treatments are somewhat more controversial as they are used to add elements to the sapphire for the purpose of improving color and thus enhancing the stone's value.[citation needed] Heat treatment is usually detectable by microscopic analysis, but diffusion isn't always detectable.
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