Ruby
Ruby is a light pink to blood red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. It is considered one of the four precious stones, together with the sapphire, the emerald and the diamond. Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.
Prices of rubies are primarily determined by color (the brightest and best "red" called Pigeon Blood Red, command a huge premium over other rubies of similar quality). After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate the stone has been treated one way or another. Cut and carat (size) also determine the price.
General
Category Mineral variety
Chemical formula aluminium oxide with chromium, Al2O3::Cr
Identification
Color Red, may be brownish or purplish
Crystal habit Varies with locality. Terminated tabular hexagonal prisms.
Crystal system Trigonal
Cleavage No true cleavage
Fracture Uneven or conchoidal
Mohs Scale hardness 9.0
Luster Vitreous
Refractive index ~1.762-1.770
Pleochroism Orangey Red, Purplish Red
Ultraviolet fluorescence red under longwave
Streak white
Specific gravity 4.0
Melting point 2050°C
Fusibility perfectly
Solubility none
Diaphaneity transparent
Category Mineral variety
Chemical formula aluminium oxide with chromium, Al2O3::Cr
Identification
Color Red, may be brownish or purplish
Crystal habit Varies with locality. Terminated tabular hexagonal prisms.
Crystal system Trigonal
Cleavage No true cleavage
Fracture Uneven or conchoidal
Mohs Scale hardness 9.0
Luster Vitreous
Refractive index ~1.762-1.770
Pleochroism Orangey Red, Purplish Red
Ultraviolet fluorescence red under longwave
Streak white
Specific gravity 4.0
Melting point 2050°C
Fusibility perfectly
Solubility none
Diaphaneity transparent
Treatments and enhancements
Improving the quality of gemstones by treating them is common yet time-consuming practice. Some treatments are used in almost all cases and are therefore considered "acceptable" practices. The most common treatment is using heat. Most if not all rubies at the lower end of the market are heat treated. Heat treatment is performed on the rough stones to improve color, remove purple tingle, blue patches and silk. These heat treatments typically occur around temperatures of 1800°C (3300°F).[1] Some rubies undergo a process of low tube heat, when the stone is heated over charcoal of a temperature of about 1300°C (2400°F) for 20 to 30 minutes. The silk is only partially broken as the color is improved.
A less acceptable treatment, and one which has gained notoriety in recent years is "Lead Glass Filling" of Rubies. By filling the fractures inside the ruby with so-called lead glass the transparency of the stone is dramatically improved making previously unsuited rubies now fit for applications in jewelry. The process is typically done in 4 steps:
The rough stones are pre-polished to eradicate all surface impurities that may affect the process
The rough is cleaned with hydrogen fluoride
The first heating process whereby no fillers are added. The heating process eradicates impurities inside the fractures. Although this can be done at temperatures up to 1400°C (2500°F) it most likely occurs at a temperature of around 900°C (1600°F) since the rutile silk is still intact
The second heating process in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful, however mostly lead-containing glass-powder is used at present. The ruby is dipped into oil, then covered with powder, embedded on a tile and placed in the oven where it is heated at around 900°C (1600°F) for one hour in an oxidizing atmosphere. The orange colored powder transforms upon heating into a transparent to yellow-colored paste, which fills all fractures. After cooling the color of the paste is fully transparent, that dramatically improves the overall transparency of the ruby.
In case a color needs to be added, the glass powder can be "enhanced" with copper or other metal oxides as well as elements such as sodium, calcium, potassium etc.
The second heating process can be repeated three to four times consecutively, even applying different mixtures.
Synthetic and imitation rubies
In 1837 Gaudin made the first synthetic rubies by fusing aluminium at a high temperature with a little chromium as a pigment. In 1847 Edelman made white sapphire by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2 and Al2O3 with a little Chromium at red heat. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.
Other processes in which synthetic rubies can be produced are through the Pulling process, flux process, and the hydrothermal process. Most synthetic rubies originate from flame fusion, due to the low costs involved. Synthetic rubies may have no imperfections visible to the naked eye but magnification may reveal curves striae and gas bubbles. The fewer the number and the less obvious the imperfections, the more valuable the ruby is; unless there are no imperfections (i.e., a "perfect" ruby), in which case it will be suspected of being artificial. Dopants are added to some manufactured rubies so they can be identified as synthetic, but most need gemmological testing to determine their origin.
Imitation rubies have also been present in the gemstone market for some time. Red spinel, red garnet and even glass have been falsely named as rubies. Imitations go back to Roman times and already in the 17th century techniques were developed to color foil red -- by burning scarlet wool in the bottom part of the furnace -- which was then placed under the imitation stone. [4] Trade terms such as balas ruby for red spinel and rubellite for red tourmaline can mislead unsuspecting buyers. Such terms are therefore discouraged from being used by many gemological associations such as the Gemological Institute of America (GIA).
Improving the quality of gemstones by treating them is common yet time-consuming practice. Some treatments are used in almost all cases and are therefore considered "acceptable" practices. The most common treatment is using heat. Most if not all rubies at the lower end of the market are heat treated. Heat treatment is performed on the rough stones to improve color, remove purple tingle, blue patches and silk. These heat treatments typically occur around temperatures of 1800°C (3300°F).[1] Some rubies undergo a process of low tube heat, when the stone is heated over charcoal of a temperature of about 1300°C (2400°F) for 20 to 30 minutes. The silk is only partially broken as the color is improved.
A less acceptable treatment, and one which has gained notoriety in recent years is "Lead Glass Filling" of Rubies. By filling the fractures inside the ruby with so-called lead glass the transparency of the stone is dramatically improved making previously unsuited rubies now fit for applications in jewelry. The process is typically done in 4 steps:
The rough stones are pre-polished to eradicate all surface impurities that may affect the process
The rough is cleaned with hydrogen fluoride
The first heating process whereby no fillers are added. The heating process eradicates impurities inside the fractures. Although this can be done at temperatures up to 1400°C (2500°F) it most likely occurs at a temperature of around 900°C (1600°F) since the rutile silk is still intact
The second heating process in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful, however mostly lead-containing glass-powder is used at present. The ruby is dipped into oil, then covered with powder, embedded on a tile and placed in the oven where it is heated at around 900°C (1600°F) for one hour in an oxidizing atmosphere. The orange colored powder transforms upon heating into a transparent to yellow-colored paste, which fills all fractures. After cooling the color of the paste is fully transparent, that dramatically improves the overall transparency of the ruby.
In case a color needs to be added, the glass powder can be "enhanced" with copper or other metal oxides as well as elements such as sodium, calcium, potassium etc.
The second heating process can be repeated three to four times consecutively, even applying different mixtures.
Synthetic and imitation rubies
In 1837 Gaudin made the first synthetic rubies by fusing aluminium at a high temperature with a little chromium as a pigment. In 1847 Edelman made white sapphire by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF2 and Al2O3 with a little Chromium at red heat. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.
Other processes in which synthetic rubies can be produced are through the Pulling process, flux process, and the hydrothermal process. Most synthetic rubies originate from flame fusion, due to the low costs involved. Synthetic rubies may have no imperfections visible to the naked eye but magnification may reveal curves striae and gas bubbles. The fewer the number and the less obvious the imperfections, the more valuable the ruby is; unless there are no imperfections (i.e., a "perfect" ruby), in which case it will be suspected of being artificial. Dopants are added to some manufactured rubies so they can be identified as synthetic, but most need gemmological testing to determine their origin.
Imitation rubies have also been present in the gemstone market for some time. Red spinel, red garnet and even glass have been falsely named as rubies. Imitations go back to Roman times and already in the 17th century techniques were developed to color foil red -- by burning scarlet wool in the bottom part of the furnace -- which was then placed under the imitation stone. [4] Trade terms such as balas ruby for red spinel and rubellite for red tourmaline can mislead unsuspecting buyers. Such terms are therefore discouraged from being used by many gemological associations such as the Gemological Institute of America (GIA).
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