The corundum family stands as a titan in the world of gemstones. It comes in almost all colours and celebrated for its exceptional hardness, vibrant colour, transparency, and luster. When it is formed in a red colour, it’s a Ruby, and when it is formed in another colour, it’s different types of sapphire gemstones.

Corundum typically forms in the Earth’s crust under extreme heat and pressure, in metamorphic rocks such as gneiss and schist, or in igneous rocks such as syenite and pegmatite. It is mainly composed of aluminum oxide, with or without trace elements such as chromium, iron, titanium, and vanadium, which give different colors during its formation. Corundum often forms a bi-pyramidal, barrel-like, or unterminated prismatic crystal.

PHYSICAL PROPERTIES / COMPOSITION

Composition—aluminum oxide

Hardness—9 on the Mohs scale

S.G. – 3.95 to 4.03

R.I. – 1.757 to 1.779

Fracture—conchoidal or irregular/uneven

Pleochroism—Yes, strong

Crystal structure—hexagonal (trigonal)

Luster—vitreous to adamantine

Transparency—Transparent to opaque

HISTORY

The name “sapphire” is derived from the Sanskrit word “Kuruvinda,” meaning “ruby.” When scientific identification did not come, by mistake, many red and blue stones were known as sapphire and ruby. Spinel was recognized as ruby for hundreds of years.

Sapphire is the state gemstone of the USA, Montana, and Queensland (Australia). Blue sapphire is the national stone of Sri Lanka, while Thailand and Myanmar honor the ruby as their national stone. Price depends on the 4C’s ( colour, cut, clarity, and carat weight), luster, treatment, geological origin, and phenomena.

ORIGIN PLACES

Corundum mineral is found worldwide, but gem corundum over 10 carats is rare, especially ruby is rarer than sapphire. Gem-quality corundum is found primarily in Sri Lanka, Myanmar, Thailand, and Madagascar. Other notable places are Australia, Vietnam, Tanzania, Mozambique, and the United States. Sri Lanka is known for its unheated sapphires, and Myanmar (formerly Burma) is globally known for its intense red rubies. Kashmir is world-famous for its royal blue sapphire with a velvety appearance.

Different sources of corundum have different properties, mainly gravity, refractive index, inclusions, and colour. Example: The reddish-brown sapphire is found in the Umba River valley, which has the highest gravity of 4.06, while the lowest-gravity ruby, at 3.8, is found in Yugoslavia. Similarly, the Sri Lankan blue sapphire has the lowest refractive index of 1.75, while the green sapphire has the highest of 1.779.

COMMON INCLUSION FOUND IN CORUNDUM

Corundum belongs to type 2 gemstones, which means visible inclusions are normal in corundum except in high-quality. Its inclusions include

Rutile needle—A hair-like inclusion can create a silky sheen effect. crossing at 60 degrees to each other.

Hexagonal growth lines—form due to growth pattern.

Fingerprints—names derived from their resemblance to a human fingerprint—are often formed by a healed fracture within the crystal.

Mineral crystal—a crystal of another mineral, typically spinel, garnet, zircon, or pyrite. These crystals can be single or in groups, which affects the stone’s transparency.

Feather—A fracture pattern that looks like a feather, often white or transparent.

Needles—thin, hair-like inclusions—can be made of different minerals.

Most corundum’s inclusions reduce its value, but hematite or rutile silk inclusions, which can make asterism or a velvety look in Kashmir sapphire, increase the cost.

Treatments or Enhancements

Almost 95 percent of sapphires are treated differently to improve clarity, luster, and optical phenomena and to improve or remove colour. Common treatments on corundum include heating, diffusion, fracture filling (flux, lead glass, or same R.I. material), irradiation, dyeing, thin film coating, etc. Thin film coating is the rarest treatment in corundum, while some treatments apply to a particular variety for a specific purpose, like lattice/beryllium diffusion, which gives a light coloured sapphire a padparadscha shade. Diffusion only applies to a star ruby or a blue sapphire.

Among all treatments of corundum, low-temperature heat treatment is the hardest to detect.

USES OF CORUNDUM

A) For industrial use

It finds application in various fields such as abrasives, laser gain medium, satellite lenses, scratch-resistant glass, space equipment, industrial bearings, and circuit board wafers, among others.

B) A Gift of Wedding Anniversaries

Ruby commemorates the 15th, 40th, 52nd, and 80th anniversaries. Sapphire is for the 5th, 45th, and 75th anniversaries.

C) Used in Jewellery

Corundum’s different types /colours are used in jewellery. Its unique bright colour and luster give a marvelous look to jewellery.

D) Used in Astrology

Corundum’s gems have a significant role in astrology. For example, ruby is associated with the sun, giving leadership qualities and confidence to the wearer. Blue sapphire is linked with the planet Saturn and protects the wearer from negative energies. Yellow sapphire, connected with the planet Jupiter, provides wisdom, clarity, and prosperity to the wearer. According to astrology, other colours of corundum give different benefits to the wearer.

E) Health Benefits

Corundum has been used to protect against many health-related issues for years. Ruby is popular for treating heart and blood-related problems, while sapphire is known for treating insomnia and vision issues.

F) Makes Decorative Items

Corundum is widely used to make many decorative items. But mainly, synthetic corundum is used for this. Synthetic corundum was first created in 1837. It has the same composition, luster, hardness, and appearance as natural, with the advantage of low price and large size availability.

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