What Is Corundum?
Corundum is a crystalline form of aluminium oxide. In its pure state, it is colourless. When trace elements enter the crystal structure, corundum becomes one of the most valuable gemstones in the world.
Ruby and sapphire are both varieties of corundum. Red corundum is called ruby. Every other colour, including blue, yellow, pink, and green, is classified as sapphire.
From a jeweller’s perspective, corundum is the backbone of fine jewellery. It defines what strength and brilliance should look like in a gemstone.
How Does Corundum Form?
Corundum forms under intense geological conditions. It develops deep within the Earth, where aluminium-rich rocks are exposed to high heat and pressure.
These conditions occur in metamorphic environments and certain igneous settings. Temperatures often exceed 700 degrees Celsius, which explains corundum’s exceptional hardness.
The presence of trace elements such as chromium, iron, and titanium during formation determines the final colour of the crystal.
Key points:
• Corundum forms under extreme heat and pressure.
• Trace elements give rise to ruby and sapphire colours.
Where Is Corundum Found?
Corundum is found on nearly every continent, but gemstone-quality material comes from specific regions.
Major sources include:
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Myanmar
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Sri Lanka
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Thailand
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Madagascar
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Australia
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Mozambique
Sri Lanka is particularly important, supplying a wide range of sapphire colours. Industry estimates suggest that around 25% of the world’s gem-quality sapphires originate there.
Many corundum crystals are discovered in river gravels, where erosion has carried them from their original rock formations.
What Are Its Properties?
Corundum’s properties are the reason it dominates both jewellery and industry.
Hardness:
It ranks 9 on the Mohs scale, second only to diamond.
Durability:
Corundum resists scratching, heat, and chemical damage.
Crystal system:
It crystallises in the trigonal system, often forming barrel-shaped crystals.
Optical qualities:
Corundum can display phenomena such as asterism and colour change.
These properties make corundum one of the most reliable gemstones for long-term wear.
Key points:
• Corundum is one of the hardest natural materials.
• Its durability makes it ideal for jewellery.
Where Did It Get Its Name From?
The name “corundum” comes from the ancient Sanskrit word kuruvinda, referring to a ruby-rich region in India.
The term passed through Greek and Latin before becoming established in modern mineralogy. This long linguistic journey mirrors corundum’s long cultural significance.
Ruby and sapphire were known and valued long before the science behind them was understood. Corundum has always been recognised, even when it was not fully explained.
Corundum Pricing
Corundum pricing varies more than almost any other gemstone category.
Typical ranges include:
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Commercial sapphires: £50–£300 per carat
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Fine sapphires: £1,000–£10,000 per carat
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Fine rubies: £5,000–£100,000+ per carat
Exceptional rubies from Myanmar have achieved prices exceeding £1 million per carat at auction.
Colour, clarity, cut, origin, and treatment all influence price. No two stones are valued in exactly the same way.
Key points:
• Ruby and sapphire pricing differ dramatically.
• Origin and colour drive the highest values.
History of Corundum
Corundum has been treasured for thousands of years. Rubies were worn by royalty in ancient India and Southeast Asia, believed to offer protection and power.
In medieval Europe, sapphires symbolised wisdom and divine favour. Clergy and monarchs wore them prominently.
By the 19th century, corundum became central to gemmology as science clarified that ruby and sapphire were the same mineral. This discovery reshaped the gemstone trade permanently.
Different Types of Corundum
Corundum is divided primarily by colour and optical effects.
Major types include:
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Ruby
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Blue sapphire
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Fancy-colour sapphires
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Star ruby and star sapphire
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Colour-change sapphire
Each type appeals to a different audience, but all share the same core properties. This consistency is one reason corundum remains so trusted.
Can It Be Lab Grown?
Yes, corundum can be lab-grown, and it has been produced synthetically for over a century.
Lab-grown corundum is chemically identical to natural material. It is widely used in jewellery, watchmaking, and industry.
In fact, over 80% of corundum used in industrial applications is synthetic, including sapphire glass for electronics and optics.
Natural corundum remains more valuable, but lab-grown stones play an important role in accessibility.
Key points:
• Synthetic corundum is widely used.
• Natural stones still command a higher value.
Is It Suitable to Be Used in Jewellery?
Absolutely. Corundum is one of the best gemstones for jewellery.
Its hardness makes it ideal for:
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Engagement rings
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Daily-wear rings
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Bracelets
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Watches
As a jeweller, corundum is a stone I recommend without hesitation. With basic care, it will last for generations.
Few gemstones combine beauty and resilience in quite the same way.
Where Is This Gemstone Used?
Beyond jewellery, corundum plays a major role in modern technology.
It is used in:
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Watch crystals
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Smartphone screens
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Scientific instruments
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Laser systems
Synthetic sapphire windows are used in space and defence technology. Industrial data shows that synthetic sapphire production exceeds 20,000 tonnes annually, dwarfing gemstone use.
Why Is This Gemstone Studied?
Corundum is studied for both scientific and commercial reasons.
Geologists use it to understand deep Earth processes. Its presence reveals pressure, temperature, and chemical conditions during rock formation.
Engineers study synthetic corundum for its strength, thermal stability, and optical clarity. These qualities are essential in high-performance environments.
Key points:
• Corundum reveals Earth’s geological history.
• Its properties drive advanced technology.
Conclusion
Corundum is not just important. It is foundational. It defines what we expect from a gemstone in terms of strength, beauty, and longevity.
As someone who has worked with gemstones for many years, I see corundum as the benchmark. Whether you admire a historic ruby, wear a sapphire ring, or tap on a sapphire screen, you are interacting with one of nature’s greatest materials.
Corundum earns its place at the very top of the gemstone world.
FAQs
Is corundum always red or blue?
No. Corundum comes in many colours. Only red is called ruby. All other colours are sapphires.
Is corundum harder than diamond?
No. Diamond ranks 10 on the Mohs scale. Corundum ranks 9.
Can corundum scratch other gemstones?
Yes. It can scratch most gemstones except diamond.
Are lab-grown sapphires real sapphires?
Yes. They are chemically and structurally identical to natural sapphires.
Why are rubies usually more expensive than sapphires?
Fine ruby is rarer, especially in large sizes and strong colour.
If you would like, I can also write a comparison between ruby, sapphire, and spinel to help readers make informed buying decisions.