Corundum is better known for its crystalline gem varieties ruby and sapphire, which are some of the most popular and valued of all gemstones. In 1725, the current name of this gemstone was derived from the Sanskrit word “kuruvinda,” and the spelling "corundum" was firstusedin 1794.  The red variant of corundum is called ruby and all other color variants, including colorless crystals, are called sapphires.

Due to its compact structure and lack of gemstone cleavage, corundum is the second hardest natural mineral after diamond. These factors, along with gem-quality corundum being quite rare, make both varieties of corundum some of the most highly desired jewelry stones. Our buying guide will tell you everything about corundum.

What is corundum?

A mineral known for its extreme hardness, corundum is a rock-forming mineral that is found in sedimentary, metamorphic, and igneous rocks. It’s an aluminum oxide with a chemical formula Al₂O₃ and a hexagonal crystal structure. The availability of beautiful transparent crystals in many different colors and its extreme hardness make it the perfect material for cutting gemstones.

Where is corundum found?

Corundum occurs in metamorphosed crystalline limestones and dolomites and in igneous rocks such as granite, pegmatite and nepheline syenite. Gem-quality corundum is usually found in placer deposits. It is mined in Sri Lanka, Thailand, Vietnam, Pakistan, Afghanistan, and India.

How to identify corundum stone?

The extreme hardness (9 on the Mohs hardness scale) and the specific gravity of corundum make it easily distinguishable from any material. It also has an unusually high density for a transparent mineral.

What are the varieties of corundum?

Pure corundum is rare and is completely colorless. Color variations arise when small amounts of metallic impurities such as chromium, iron, and titanium substitute for aluminum in the structure. The popular varieties include:

• Ruby: The name ruby originates from the Latin word “Ruber,” meaning red. Any gem-quality corundum with a deep red color is called a ruby.Rubies get their red color due to the presence of chromium.

• Padparadscha Sapphire: This rare orange-pink variety owes its unique color to both iron and chromium impurities.

• Sapphire: The name sapphire has its origin from the Latin word “Sapphires,” which means blue. It is also thought to have been used in ancient times to refer to lapis lazuli. The name sapphire is a loose term used for all colors of corundum except red. Sapphires can be blue, pink, green, yellow, violet, purple, orange, brown, white, gray, black, and colorless. Theyget their unique color from Fe²⁺, Fe³⁺, and Ti⁴⁺inclusions.

• Emery: It is a black variety of corundum used as an abrasive and for some electrical purposes. The black color is caused by a mixture of magnetite, hematite, and spinel. 

The 3C’s of Corundum:

Color : 

The pinkish-red, medium to dark red variants of corundum are called ruby. All the other available color variants(gray, blue, blue-green, green, violet, purple, orange, yellow, yellow-green, brown, golden amber, peachy pink, pink, and black), including colorless and white crystals,are called sapphire.  They usually have vitreous to adamantine luster with a white streak.

Cut:

As a result of its hardness and durability, corundum is commonly used in jewelry designs that see high traffic or the likelihood of impacts. Corundum should be cut according to its color. The red variant should be cut as if it were ruby, while blue variants should be cut according to the uses of sapphire.

Clarity:

Most cut rubies and sapphires contain inclusions. Eye-visible inclusions are lesser desirable than "eye-clean" stones. In some stones, inclusions can make the stone weaker and lead to breakage. Desirable inclusions and fine rutile needles throughout the crystal cause asterism (a phenomenon where gems exhibit star-like patterns when cut en cabochon) and provide a silky texture.

Corundum Treatments:

Corundum generally undergoes the following treatments –

• Heat Treatment - 

Heat treatment causes a change in the color of corundum by changing the oxidation state of the impurities present. For example, the reduction of Fe causes the color to change in a variety of sapphires via charge transfer. Stones that are too deeply blue can be lightened by oxidation (the opposite process of reduction) of Fe. Sometimes, heat treatment improves the depth of color by causing dissolution of inclusions and diffusion of impurities into the surrounding corundum. Heat treatment can also enhance the clarity of stones that look cloudy by dissolving the inclusions. Local color concentrations can be removed using heat, which allows the color-causing impurity to be more evenly distributed through the crystal. Gems that have undergone heat treatment can be detected by the modified natural inclusions.

• Diffusion Treatment - 

It is done to enhance the blue color by placing Fe and Ti oxide powders in contact with the faceted gem and allowing Fe and Ti to diffuse into the surface of the stone. Diffusion treatment is done to faceted stones and is not always obvious by inspection, but can be detected through concentration of color at facet junctions, and a modified the refractive index. 

How to clean corundum?

Corundum is the ideal choice for regular wear, but it should be protected from scratches and sharp blows. Warm soapy water and a soft cloth are the safest choices for cleaning. Chemicals, heat, and ultrasonic cleaners should be avoided if the gemstone is dyed. Corundum that is dyed or has a scratch or fracture should be cleaned gently with a damp cloth. Corundum rings, necklaces, bracelets, and earrings should be stored carefully in a box.

Uses of Corundum:

• The extreme hardness of corundum makes it an excellent abrasive. Crushed corundum is processed to get rid of impurities and then screened to produce uniformly sized granules. These granules are used by grinding media for polishing compounds, sandpapers, grinding wheels, and other cutting applications.
• "Emery boards" are a manicure and nail-care product made by gluing abrasive papers made of emery to a thin piece of cardboard.
• Ruby and sapphire are gem value corundum; these popular gems have high demand and have been mined all over the world for thousands of years.
• Drilling a small hole into a tiny piece of corundum allows it to be used as a smooth-running, long-life bearing.
• Many lasers use synthetic corundum. The first laser was a "ruby laser."

Metaphysical Properties of Corundum:

• Promotes insight into the unknown.
• Increases wisdom and helps make wise choices.
• It is believed to subdue emotional tumult by relieving tension and releasing anger in a positive manner.
• Reduces the presence of negative thoughts and enhances intuition.
• While corundum is not a birthstone in itself (as ruby and sapphire are), corundum has been linked with the zodiac sign Sagittarius.

Where to buy corundum from?

At GemsforJewels, we offer corundum gemstone in various beautiful shades. These stones can be bought loose, as cabochons, or even as strands; there are umpteen alluring varieties to choose from. We also provide different shapes to pick from, including beads, briolettes, tear drop, square, and many other exciting shapes. Corundum and its gem variants, including ruby and sapphire set in rings, necklaces, and earrings, will be loved by your customers.