Mohs Hardness 9: Corundum — Ruby and Sapphire Hardness

Corundum at Mohs 9 is the second hardest natural mineral and the hardest colored gem material in common use. Ruby and sapphire are both gem varieties of corundum, differing only in color. Their Mohs 9 hardness, combined with excellent toughness and stability, makes them the most practical colored gems for everyday jewelry wear.

---

What Is Corundum?

Corundum is aluminum oxide (Al2O3) crystallizing in the trigonal system. Pure corundum is colorless. Trace amounts of chromium produce the red color of ruby. Trace amounts of iron and titanium, or other elements, produce the blue, yellow, pink, orange, green, and other colors of sapphire. Despite their different colors and names, ruby and sapphire are the same mineral species with the same hardness, toughness, and basic care requirements.

---

Why Corundum Is Mohs 9

Corundum's hardness comes from its tightly packed aluminum and oxygen atoms bonded in a hexagonal close-packed structure. The aluminum-oxygen bonds are strong and directional, creating a crystal that resists scratching from virtually all natural materials except diamond. The structure has no cleavage, only parting along certain planes, which contributes to corundum's excellent toughness as well as its high hardness.

---

Hardness Compared to Diamond

While corundum is Mohs 9 and diamond is Mohs 10, the absolute hardness difference is enormous. In Vickers hardness measurements, diamond is approximately 4 times harder than corundum. This means that diamond scratches corundum easily, but corundum scratches everything else in the gem world except diamond. For practical jewelry wear, corundum's Mohs 9 hardness is more than sufficient: nothing in everyday life except diamond will scratch a ruby or sapphire.

---

Toughness of Corundum

Corundum has excellent toughness as well as high hardness. It has no true cleavage, only parting along rhombohedral and basal planes that is much less pronounced than true cleavage. This means corundum does not split easily from impact the way diamond or topaz can. Ruby and sapphire can withstand the knocks and impacts of everyday wear without chipping under normal conditions. This combination of high hardness and excellent toughness makes corundum the ideal gem for engagement rings and other daily wear jewelry.

---

Ruby vs Sapphire: Same Hardness, Different Colors

Ruby is defined as red corundum, with the red color caused by chromium. All other gem-quality corundum colors are called sapphire, including blue (the most common and valuable), yellow, pink, orange, green, purple, and colorless. Padparadscha sapphire is a rare pinkish-orange variety. Star ruby and star sapphire show asterism, a six-rayed star caused by needle-like inclusions of rutile.

All varieties of gem corundum have the same Mohs 9 hardness and the same excellent toughness. Care requirements are identical across all colors.

---

Treatments and Their Effect on Care

The majority of commercial ruby and sapphire has been heat-treated to improve color and clarity. Heat treatment is stable and permanent, and heat-treated corundum can be cared for exactly like untreated corundum. Warm soapy water, soft brush cleaning, and ultrasonic cleaners are all appropriate.

Some lower-quality rubies are fracture-filled with glass to improve clarity. Glass-filled ruby requires much more careful care: avoid ultrasonic cleaners, steam, and chemical cleaners, as these can damage or displace the glass filler. Always ask about treatments when purchasing ruby.

---

Caring for Ruby and Sapphire

Ruby and sapphire are among the easiest gems to care for. Clean with warm soapy water and a soft brush. Ultrasonic cleaners are safe for heat-treated stones without fracture filling. Store separately from diamond, which can scratch them. Annual prong inspection is recommended for rings worn daily. Ruby and sapphire are suitable for virtually any jewelry application including rings, bracelets, and other pieces subject to daily wear and impact.

---

Frequently Asked Questions

Is ruby harder than sapphire?

No. Ruby and sapphire are both corundum and have identical hardness of Mohs 9. The difference is only in color, caused by different trace element impurities.

Can sapphire scratch ruby?

Yes. Since ruby and sapphire are the same mineral at the same hardness, they can scratch each other. Store ruby and sapphire pieces separately to prevent contact damage.

Why is corundum used in industrial applications?

Synthetic corundum (aluminum oxide) is used as an abrasive under the name emery or alumina. It is used to grind and polish metals, glass, and other materials. Synthetic ruby and sapphire are used in watch bearings, scientific instruments, and as substrates for semiconductor devices.

---

Conclusion

Corundum at Mohs 9 is the hardest colored gem material in common use and the most practical choice for everyday colored gem jewelry. Its combination of Mohs 9 hardness, excellent toughness, and excellent stability makes ruby and sapphire ideal for engagement rings, daily wear rings, and any jewelry application requiring maximum durability. Only diamond surpasses corundum in hardness among natural gem materials.