Heated gemstones are less effective



sapphire is a variety of the mineral corundum. All colorless and brightly colored varieties are assigned to sapphire with the exception of red ruby. In a narrower sense, the term today mainly refers to the blue variants, which still range from sky blue to dark blue that goes into black and can vary in color depending on the incidence of light.


The word sapphire is derived from the Greek word σάπφειρος sappheiros "Blue", which is from Hebrew sappir is derived. There are outdated or incorrect trade names: Oriental aquamarine (greenish blue sapphire), Oriental Huazinth ü (pink sapphire), Oriental emerald (green sapphire) and Oriental topaz (yellow sapphire).

Special properties

Like all corundum, sapphire also crystallizes in the trigonal crystal system with the chemical composition Al2O3 and develops predominantly double-sided, barrel-shaped, six-sided pyramidal and prismatic crystals. Chemical resistance is also typical of corundum. For example, sapphire is insoluble in acids and only melts at a temperature of 2040 ° C.

Color and optical effects

Sapphires contain small amounts of Fe as coloring substances2+ or Ti3+ (blue), Fe3+ (yellow and green), cr3+ (pink) and / or V4+ (purple, orange together with chrome and iron). The colorless leuco sapphire, on the other hand, does not contain any additives.

The classic gemstone sapphire is an intense but not too dark blue ("Kashmir sapphires"). A specialty among the gemstone sapphires is the so-called predominantly from Asia Padparadscha, a pink to orange-colored variant, the name of which is derived from the Sinhalese word for lotus blossom. Padparadschas originally come from Sri Lanka, but are now often color-treated and can then come from all over the world.

Also in demand are those awarded with the asterism optical effect Star sapphires. Due to the oriented embedded rutile needles, a more or less perfect, six-ray, star-shaped reflection can be seen.

Education and location

Until recently, the most important producers of sapphires were Sri Lanka and India; today the gemstones also come from the USA, Australia and Nigeria. Sapphires from Madagascar, more precisely Ilakaka, are considered to be of very high quality, but are usually declared as coming from Sri Lanka because they achieve higher prices. Production in Australia has decreased significantly in recent years.

Synthetic production and chemical-technical treatment

Synthetic sapphires have been produced in perfect quality and in almost unlimited sizes since 1910. Colorless, synthetic sapphires are sometimes sold under the misleading trade name “Diamandite” or “Diamondite” and are used as diamond imitations.

The sapphires that are commercially available as “natural” are often heat-treated. The heat treatment can be used both to change the color and to increase the clarity of a sapphire. With light heat treatment, microscopic structures such as rutile needles (“silk”) are retained; if heated to a great extent, these natural micro-inclusions are destroyed. Superficial cracks or small bumps are often covered by melting borax and lead crystal glass or by treating them with oil.

Blue sapphires in particular can also be achieved using a diffusion process. Here the stone is heated to 1800 ° C together with beryllium powder. Well-known suppliers also use treated sapphires, sometimes including the controversial diffusion treatment, but without individual declarations (e.g. Tiffany & Co.).

Use as a gem stone

Sapphires are mainly processed into gemstones. Transparent stones of high quality (as few inclusions as possible) are given a facet cut, opaque and especially those with asterism, on the other hand, are made into cabochons to emphasize the star effect.

The largest sapphire ever cut is the “Star of India” with a weight of 563.35 carats. The stone found in Sri Lanka was donated to the American Museum of Natural History by John Pierpont Morgan in 1901 and can be viewed there.

Other uses

In addition to its use as a gemstone, the sapphire was used as the tip of the pickup needle in record players of the 1950s and 1960s. Because of its high hardness and abrasion resistance, sapphire is also used as a guide for wire EDM machines. Compared to the more solid diamond, it offers considerable cost advantages despite its shorter service life.

Synthetic monocrystalline sapphire substrates are the most important starting material for the artificial crystal growth (epitaxy) of gallium nitride, a substance that is used in blue, white and green LEDs and blue semiconductor lasers.

With the addition of titanium as a laser ion, synthetic sapphires are themselves an important component for laser applications, in particular for tunable lasers in the wavelength range from 700 to around 1000 nanometers; these are known as titanium: sapphire lasers.

Synthetic sapphires with a diameter of up to 75 centimeters are used for the windows of reconnaissance aircraft, anti-aircraft missiles or spacecraft that are exposed to extreme loads.

In special cases, sapphire is also used in scientific instruments in space travel, for example in the Genesis mission.

Because of its high thermal conductivity of 40 W / (m • K) at a normal temperature of 25 ° C compared to other insulating materials, discs made of this material are used in scientific experiments, for example when effective cooling or precise temperature control is required the observation must be made through a transparent medium. As the temperature rises, however, the thermal conductivity decreases and is still 12 W / (m • K) at 400 ° C and only 4 W / (m • K) at 1200 ° C. A temperature decrease, on the other hand, causes a sharp increase in thermal conductivity, which reaches a value of 10,000 W / (m • K) at a temperature of –200 ° C, making the sapphire very suitable for low-temperature experiments.

Glasses made of synthetic sapphire are used in high-quality wristwatches.

In the case of semiconductors, there is what is known as silicon-on-sapphire technology.


The sky blue variant is commonly associated with qualities such as calm, purity, and peace. There is no scientific evidence for the alleged physical or psychological effects.

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Authors: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0