When you are choosing a gemstone to wear every day, durability matters as much as color. Sapphire and ruby — both varieties of the mineral corundum — are the hardest natural gemstones after diamond, which is precisely why they have been set into engagement rings, signet rings, and heirloom jewelry for centuries. This guide explains what makes corundum so resilient, the crucial difference between hardness and toughness, and how to keep a sapphire or ruby looking flawless for generations.
What Corundum Is, and Why It Is So Hard
Sapphire and ruby are the same mineral: crystalline aluminum oxide, known to gemologists as corundum. When trace chromium turns the stone red, we call it ruby; in every other color — blue, pink, yellow, teal, purple, even colorless — the same mineral is called sapphire. The properties that govern durability are therefore identical across the two.
Corundum's hardness comes from its tightly packed crystal lattice, in which aluminum and oxygen atoms are bound by strong, evenly distributed bonds. There are no weak planes running through a well-formed corundum crystal, so the structure resists both scratching and the kind of stress that splits softer minerals. This atomic architecture is what allows a sapphire to survive decades of daily contact with keys, countertops, and car doors without losing its polish.
Mohs Hardness: Where Sapphire and Ruby Rank
The Mohs scale ranks mineral hardness from 1 (talc) to 10 (diamond) based on which material can scratch another. Corundum sits at 9, second only to diamond and harder than virtually every other gem on the market. It is important to know that the scale is not linear: the gap between corundum at 9 and diamond at 10 is far wider than the gap between, say, quartz at 7 and topaz at 8.
What makes a hardness of 9 so practical is the nature of everyday dust. Airborne grit is largely quartz, which has a hardness of about 7. Because corundum is meaningfully harder than quartz, ordinary dust cannot abrade it — a major reason sapphire and ruby keep their bright polish while softer stones gradually dull. For comparison, here is where common colored gems fall:
- Diamond — 10
- Sapphire and ruby (corundum) — 9
- Spinel — 8
- Emerald and aquamarine (beryl) — 7.5 to 8
- Garnet — 6.5 to 7.5
- Tanzanite — 6 to 7
- Opal — 5.5 to 6.5
Hardness Is Not the Same as Toughness
Buyers often use “hard” and “tough” interchangeably, but to a gemologist they mean different things. Hardness is resistance to scratching; toughness is resistance to chipping, cracking, and breaking under impact. A material can score high on one and lower on the other — diamond, for instance, is the hardest gem yet has cleavage planes along which a sharp blow can split it.
Corundum performs well on both counts. It has no cleavage and good overall toughness, so a sapphire or ruby is far less likely to chip than an emerald or an opal. This combination of high hardness and solid toughness is exactly why corundum is considered one of the few colored stones genuinely suited to everyday rings.
One caveat: treatments and the stone's own internal character affect toughness. A heavily fractured stone, or a ruby whose cavities have been filled with lead glass, is more vulnerable to chipping and to damage from heat and chemicals. This is one more reason to buy stones whose treatments are fully disclosed on a reputable laboratory certificate.
What This Means for Everyday Jewelry
For an engagement ring or a piece intended for daily wear, corundum is close to ideal. A sapphire center stone will shrug off the bumps and abrasion that would scuff a softer gem, and a ruby solitaire offers the same resilience with the romance of red. Designers working in production also value this durability, because calibrated corundum survives the rigors of setting, sizing, and repair far better than fragile alternatives.
If you are selecting a stone for a ring, you can browse our full range of sapphires or our ruby solitaires with confidence that the material itself is built to last. The main practical advice is simply to choose a secure setting and, for very active wearers, to favor stones without large surface-reaching fractures.
How to Care for a Sapphire or Ruby
Caring for corundum is refreshingly simple. Warm water, mild dish soap, and a soft brush remove the oils and film that dull a stone's sparkle between professional cleanings. Because corundum is chemically stable, it is unaffected by everyday soaps, lotions, and cosmetics.
Ultrasonic cleaners are generally safe for untreated and conventionally heated stones, but should be avoided for fracture-filled, glass-filled, or fissure-prone gems, where vibration and heat can worsen existing breaks. Store each piece separately so a harder stone — or another sapphire — cannot scratch your other jewelry, since at hardness 9 corundum can mark almost anything it touches except diamond.
At Thai Gems, we have cut and supplied sapphire and ruby from our Bangkok workshop for over 70 years, and durability is one of the first things we explain to buyers choosing a stone for daily wear. Explore our certified sapphire and ruby collections at thaigems.com, or contact us for trade pricing, custom cutting, and full treatment disclosure on every stone.