What is Corundum? Unveiling the Gem Behind Rubies and Sapphires

Corundum is an aluminum oxide (Al₂O₃) gemstone with a Mohs hardness of 9; it resists scratches and wear, second only to diamond.

Exactly what Is Corundum, Anyway?

Corundum is an aluminum oxide (Al₂O₃) gemstone with a Mohs hardness of 9; it resists scratches and wear, second only to diamond.

The intense geological heat and pressure over millions of years have made it one of the most resilient gemstones.

The word “corundum” comes from Tamil, a language of South Asia, reflecting its historical ties to India’s gem-rich regions.

The gem forms deep within the Earth over a million years and develops vibrant color and a distinctive character that make it one of the most favored gem varieties in jewelry design.

As a jewelry designer, I’m drawn to the red rubies and the blue sapphires. Despite their differences, both forms belong to the corundum mineral family, a mineral that’s loved by jewelers and enthusiasts at first sight.

Corundum Gem Varieties

Gem Material	Composition and type	Mohs hardness	Typical jewelry use
Corundum (ruby/sapphire)	Aluminum oxide; colored gemstone family ​	9 ​	Engagement rings, fine rings, earrings, bracelets, high‑end pieces ​
Diamond	Durable carbon-based stone	10 ​	Engagement rings, high‑wear rings, prestige jewelry ​
Emerald	Beryl colored by chromium/vanadium ​	7.5–8 ​	Dress rings, pendants, earrings need less care due to durability
Quartz (amethyst, citrine)	Silicon dioxide; common colored stones ​	7 ​	Fashion jewelry, beads, lower‑stress settings (pendants, earrings) ​

Ruby vs. Sapphire: Sisters Under One Name

The magic of corundum lies in its versatility; add chromium, and it becomes the fiery ruby, a long-standing symbol of passion and luxury; add titanium or iron, and it produces sapphires ranging from sky-blue to pastel pink and green.

Even the rare “Padparadscha” (pink-orange) sapphire falls under this family. Primarily mined in Sri Lanka, where its color is likened to a lotus flower, this rare variety of corundum is named after the Sinhala term for “aquatic flower.” Due to this corundum variety’s scarcity, it is highly valued by collectors and connoisseurs

Corundum Durability for Artisans

The hardness of corundum enables jewelers to create fine gemstone settings and pendants that resist chipping and discoloration, keeping pieces wearable across generations.

Timeless Appeal Across Cultures

Rubies and sapphires have been worn by nearly every civilization. Myanmar’s (Burma) rubies are legendary, while Sri Lanka’s Ceylon sapphires have decorated royal jewelry for centuries.

Today, we combine corundum with pearls or metals to create looks that feel both timeless and modern.

The next time you hold a ruby or sapphire, remember the ancient journey that shaped it, one of long geological evolution followed by skilled human artistry.

Whether you’re wearing ruby or sapphire as a bold statement piece or an understated charm, you’ll look beautiful and feel confident because this stone is built to last.

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