Priya Nair
What the Mohs Scale Actually Measures
The Mohs scale is a simple ranking of scratch resistance, devised by the German mineralogist Friedrich Mohs in 1812 and still in daily use because it answers the question jewellers are asked most often: will this stone survive being worn? It runs from 1 (talc, soft enough to be scratched by a fingernail) to 10 (diamond, which scratches everything else and is scratched by nothing you will meet in ordinary life). A mineral with a higher number will always scratch one with a lower number.
Two things are worth knowing before you use it to choose a piece. First, the scale is not linear: diamond at 10 is not ten times harder than talc at 1 but many hundreds of times harder, and the step between 9 and 10 is far wider than the step between 1 and 2. Second, and more importantly, it tells you only what will graze a stone's surface. It says nothing about how that stone copes with a sharp knock.
Common Jewellery Stones by Scratch Resistance
Here is roughly where the stones you will meet in a jeweller's window sit on the scale.
- Under 6 — handle with care: amber (2–2.5), pearl (2.5–3), opal (5.5–6.5), turquoise and lapis lazuli (5–6), moonstone (6–6.5).
- 6 to 7 — the middle ground: tanzanite (6–7), peridot (6.5–7), jade (6–7), garnet (6.5–7.5) and amethyst, along with every other variety of quartz (7).
- 7.5 to 8.5 — genuinely hard-wearing: tourmaline (7–7.5), emerald and aquamarine (7.5–8), topaz (8), spinel (8), cubic zirconia (8–8.5).
- 9 and above — a lifetime of daily wear: sapphire and ruby (9), moissanite (9.25), diamond (10).
You can see straight away why sapphire has become the fashionable alternative to diamond in engagement rings. It sits a single step below diamond, shares diamond's crystal family, and costs a fraction as much.
Hardness Is Not the Same as Toughness
This is the distinction that catches people out. Hardness resists scratching. Toughness resists breaking, chipping and cleaving. A stone can be hard and still fragile, and several popular gems are exactly that.
Emerald sits at 7.5–8, comfortably above quartz, yet most emeralds are threaded with internal fractures that make them prone to chipping along a facet edge. Tanzanite has one direction of perfect cleavage, so a knock at the wrong angle can split it cleanly. Diamond itself, the hardest material we know, will cleave along its grain if struck properly — which is why a jeweller taps a diamond gently, not confidently. Opal is a different case again: not especially soft, but water-bearing and heat-sensitive, so it cracks where it will not scratch.
Jade proves the opposite point. At only 6–7 it is one of the toughest jewellery materials there is, because its interlocking crystal structure spreads impact rather than letting it travel. Toughness is why a thin jade bangle can survive a lifetime of knocks that would shatter a thicker, harder stone.
Matching a Stone to the Jewellery You Actually Wear
Where a stone sits on the body matters as much as its Mohs number. Exposure to knocks rises sharply as you move down from ears to hands.
- Pendants, brooches and drop earrings rarely meet a hard surface. Opal, pearl, amber and moonstone are all realistic here, provided you keep them away from perfume and hairspray.
- Stud earrings are slightly more exposed, but 6 and above is comfortable for most wearers.
- Bracelets and bangles are the most punishing items in the jewellery box, knocking against desks, doors, car doors and shopping trolleys all day. Choose 7 or above, and accept that a stone at 7 will slowly haze over years of it.
- Rings take the worst of it, particularly on the dominant hand. An engagement ring or wedding band worn every day is best set with sapphire, ruby, moissanite or diamond.
Your own week matters too. If you garden, remember that soil contains quartz at 7 — enough to scratch opal, pearl, turquoise and tanzanite, and to leave a dull film across anything softer than 7. Swimming will not scratch a stone, but chlorine and salt water attack the metals in a setting and can damage porous gems such as pearl, opal and amber. Gym sessions, decorating and moving house are the three occasions when most ring damage happens.
Practical Habits That Follow From All This
- Take rings off for gardening, DIY, cleaning and swimming. A small dish by the kitchen sink is more useful than any protective coating.
- Store pieces separately in soft pouches. A loose diamond in a jewellery box will scratch a sapphire, a gold band and an opal with equal enthusiasm.
- Clean hard stones with warm water and a drop of washing-up liquid, using a soft brush. Skip the ultrasonic cleaner for emerald, opal, pearl, turquoise and tanzanite.
- Have settings checked about once a year. Most lost stones come from worn claws rather than soft gems.
- If a softer stone is going to be worn daily, choose a protected setting. A bezel or a halo shields the edges where chips begin, and does much of the work that hardness alone cannot.
- If you stack rings, put plain metal bands on the outside so they take the wear, and keep harder stones from rubbing against softer ones.
None of this is meant to steer you away from a stone you love. A well-cared-for opal or pearl will outlast its owner; it simply asks to be taken off before the washing-up. Buy the gem you want to look at, then let its Mohs number tell you how to look after it.
