SAPPHIRE[GEM]

Listing description:

Sapphire  is a gemstone variety of the mineral corundum, an aluminium oxide (α-Al₂O₃), when it is a color other than red or dark pink, in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Pink-orange sapphires are also called padparadscha.

Detailed description:

Pure chromium is the distinct impurity of rubies. However, a combination of e.g. chromium and titanium can give a sapphire of a color distinct from red.

Sapphires are commonly worn as jewellery. Sapphires can be found naturally, by searching through certain sediments or rock formations, or they can be manufactured for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires (and of aluminum oxide in general), sapphires are used in some non-ornamental applications, including infrared optical components, such as in scientific instruments; small, high-durability windows (also used in scientific instruments); wristwatch crystals; and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (most of which are integrated circuits).

Natural sapphires

The sapphire is one of the two or three gem-varieties of corundum, with another one being the red or deep pink ruby. Although blue is their most well-known hue, sapphires are made up of any color of corundum except for red (red ones are called rubies). Sapphires may also be colorless, and they are also found in shades of gray and black.

The cost of natural sapphires varies depending on their color, clarity, size, cut, and overall quality - as well as their geographic origin, oddly enough. Significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, Madagascar, East Africa, and in North American at a few locations, such as at "Gem Mountain", and in or near the Missouri River in the region around Helena, Montana.^[2] Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.^[3]

Blue sapphire

Color in gemstones breaks down into three components: hue, saturation, and tone. Hue is most commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness or "colorfulness" of the hue, and tone is the lightness to darkness of the hue.^[4] Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness).

Blue sapphires are evaluated based upon the purity of their primary hue. Purple, violet, and green are the most common secondary hues found in blue sapphires.^[5] Violet and purple can contribute to the overall beauty of the color, while green is considered to be distinctly negative. Blue sapphires with up to 15% violet or purple are generally said to be of fine quality. Blue sapphires with any amount of green as a secondary hue are not considered to be fine quality. Gray is the normal saturation modifier or mask found in blue sapphires. Gray reduces the saturation or brightness of the hue and therefore has a distinctly negative effect.^[5]

The color of fine blue sapphires can be described as a vivid medium dark violet to purplish blue where the primary blue hue is at least 85% and the secondary hue no more than 15% without the least admixture of a green secondary hue or a gray mask.^[4]

Fancy color sapphire

Yellow and green sapphires are also commonly found. Pink sapphires deepen in color as the quantity of chromium increases. The deeper the pink color the higher their monetary value as long as the color is trending towards the red of rubies.

Sapphires also occur in shades of orange and brown, and colorless sapphires are sometimes used as diamond substitutes in jewelry. Padparadscha sapphires often draw higher prices than many of even the finest blue sapphires. Recently, more sapphires of this color have appeared on the market as a result of a new artificial treatment method that is called "lattice diffusion".^[6][7]

Padparadscha

Padparadscha is a pink-orange corundum, with a low to medium saturation and light tone, originally being mined in Sri Lanka, but also found in deposits in Vietnam and Africa; Padparadscha sapphires are very rare and highly valued. The name derives from the Sinhalese word for lotus blossom.^[8] Along with rubies, they are the only type of corundum to be given their own name instead of being called a particular colored sapphire. Padparadscha used to be a subvariety of ruby.

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, a mineral composed primarily of titanium dioxide^[10]) that cause the appearance of a six-rayed "star"-shaped pattern when viewed with a single overhead light source.

The Black Star of Queensland is believed to be the largest star sapphire that has ever been mined, and it weighs 733 carats.^[11] The Star of India (weighing 563.4 carats) is thought to be the second-largest star sapphire, and it is currently on display at the American Museum of Natural History in New York City. The 182-carat Star of Bombay, located in the National Museum of Natural History, in Washington, D.C., is an example of a blue star sapphire. The value of a star sapphire, however, depends not only on the weight of the stone but also the body color, visibility and intensity of the asterism.

Color change sapphire

A rare variety of sapphire, known as color change sapphire, exhibits different colors in different light. Color change sapphires are blue in outdoor light and purple under incandescent indoor light; they may also be pink in daylight to greenish under fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. While color change sapphires come from a variety of locations, the gem gravels of Tanzania is the main source.

Certain synthetic color-change sapphires are sold as “lab” or “synthetic” alexandrite, which is accurately called an alexandrite simulant (also called alexandrium) since the latter is actually a type of chrysoberyl—an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire).

Source of color

Red rubies are corundum which contain chromium impurities that absorb yellow-green light and result in deeper ruby red color with increasing content.^[12] Purple sapphires contain trace amounts of vanadium and come in a variety of shades. Corundum that contains ~0.01% of titanium is colorless. If trace amounts of iron are present, a very pale yellow to green color may be seen. If both titanium and iron impurities are present together, however, the result is a magnificent deep-blue color.^[13]

Unlike localized ("interatomic") absorption of light which causes color for chromium and vanadium impurities, blue color in sapphires comes from intervalence charge transfer, which is the transfer of an electron from one transition-metal ion to another via the conduction or valence band. The iron can take the form Fe²⁺ or Fe³⁺, while titanium generally takes the form Ti⁴⁺. If Fe²⁺ and Ti⁴⁺ ions are substituted for Al³⁺, localized areas of charge imbalance are created. An electron transfer from Fe²⁺ and Ti⁴⁺ can cause a change in the valence state of both. Because of the valence change there is a specific change in energy for the electron, and electromagnetic energy is absorbed. The wavelength of the energy absorbed corresponds to yellow light. When this light is subtracted from incident white light, the complementary color blue results. Sometimes when atomic spacing is different in different directions there is resulting blue-green dichroism.

 Treatments

Sapphires may be treated by several methods to enhance and improve their clarity and color.^[14] It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires in air to temperatures between 500 and 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. Upon heating, the stone becomes a more blue in color but loses some of the rutile inclusions (silk). When high heat temperatures are used, the stone loses all of the silk and becomes clear under magnification.^[15] Evidence of sapphire and other gemstones being subjected to heating goes back to, at least, Roman times.^[16] Un-heated stones are quite rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".

Synthetic sapphire

In 1902, the French chemist Auguste Verneuil developed a process for producing synthetic sapphire crystals.^[21] In the Verneuil process, named for him, fine alumina powder is added to an oxyhydrogen flame, and this is directed downward against a mantle.^[22] The alumina in the flame is slowly deposited, creating a teardrop shaped "boule" of sapphire material. Chemical dopants can be added to create artificial versions of the ruby, and all the other natural colors of sapphire, and in addition, other colors never seen in geology. Artificial sapphire material is identical to natural sapphire, except it can be made without the flaws that are found in natural stones. The disadvantage of Verneuil process is that the grown crystals have high internal strains. Many methods of manufacturing sapphire today are variations of the Czochralski process, which was invented in 1916. In this process, a tiny sapphire seed crystal is dipped into a crucible made of the precious metal rhodium, containing molten alumina, and them slowly withdrawn upward at a rate of one to 100 mm per hour.

PRICE

$39910992/KG OR $18141360/IB

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