The Sapphire
Sapphire is a precious stone of rare elegance, renowned for its exceptional hardness and its vast palette of colors. Formed in the depths of the earth over millennia, this precious corundum can be found in various shades and has always impressed with its charm and timeless refinement.
Sapphire is one of the most valuable varieties of corundum, a family of minerals composed of aluminum oxide. This gemstone is best known for its deep blue hues, but it can also occur in many other colors, with the exception of red (reserved for ruby).
Other colored sapphires exist in shades of pink, yellow, green, purple, and even colorless hues (called leuco-sapphires). The characteristic blue color of sapphire is due to the presence of iron and titanium, while elements such as chromium, magnesium, or vanadium are responsible for the other fanciful hues.
The most famous sapphire deposits are located in Sri Lanka, Madagascar, Thailand, Australia, Myanmar, and Kashmir, whose stones are particularly prized for their velvety blue.
In its natural state, sapphire often contains inclusions, in the form of rutile hairs, which can influence its value. These natural characteristics are sometimes attenuated by heat treatments intended to improve the transparency and intensity of the color. Heat treatment is commonly practiced and accepted in the gemstone trade, although it must be indicated at the time of sale.
Sapphire is a highly prized gemstone throughout the world, particularly appreciated in jewelry for its beauty and durability. Sapphires of an intense and pure blue, without visible inclusions and weighing more than 5 carats, can fetch high prices on the market.
Some sapphires, particularly those from Sri Lanka, exhibit an asterism phenomenon, producing a rare star effect that is highly sought after in jewelry and among collectors.
Synthetic sapphires, created in the laboratory by processes such as the Verneuil method or the hydrothermal process, have the same physical and optical characteristics as natural sapphires. Only experienced gemologists can usually distinguish them from natural stones.
Sapphire can also be treated with beryllium, a process where the stone is heated with this element, which penetrates deep into the gem, altering the color more intensely and lastingly than simple heat treatment. This treatment is commonly used to produce sapphires with vibrant hues, such as yellow, orange, or orange-pink (often called “padparadscha” sapphires). However, because of its profound and irreversible impact on the stone, beryllium treatment must be mentioned when selling. In addition, sapphires treated with beryllium are generally less expensive than those treated by simple heating, and their value remains significantly lower than that of natural, untreated sapphires. To ensure the authenticity of your sapphire, it is recommended to turn to a trusted gemologist.
Physical properties of sapphire:
Each gemstone has unique characteristics, such as color, clarity, and hardness, that influence its brilliance and durability. These properties are the result of the stone's chemical composition and crystal structure, which contribute to its appeal in jewelry and fine jewelry.Here are detailed resources on this topic.
Chemical composition: Aluminum oxide (Al₂O₃), with traces of iron, titanium, chromium, or magnesium that determine color variations.
Hardness: 9 on the Mohs scale (sapphire is one of the hardest minerals after diamond).
Density: About 3.95 to 4.03 g/cm³.
Crystal system: Rhombohedral (trigonal).
Cleavage: None, although stress fractures may occur.
Fracture: Conchoidal to irregular, edges are often slightly chipped.
Specific gravity: Varies from 3.95 to 4.03.
Melting temperature: About 2040 °C.
Optical properties of sapphire:
Transparency: Transparent to translucent.
Refractive Index: Between 1.762 and 1.770.
Birefringence: 0.008 to 0.010, meaning that the sapphire may exhibit a slight double refraction effect.
Brilliance: Vitreous, with a beautiful luminosity.
Color: Primarily blue, but can appear pink, yellow, green, purple, colorless, etc., depending on the inclusions present.
Fluorescence: Variable depending on color and trace elements, but most blue sapphires are non-fluorescent.
Saturation: The saturation and intensity of the color directly affects its value, with deep, velvety blues being particularly sought after.
Pléochroïsme: Present, blue sapphire can exhibit shades ranging from blue to purple depending on the viewing angle.
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