Why a hardness number can mislead you
Did you see a gemstone’s Mohs number, assume it meant the stone was tough, and then watch a highly rated stone chip? Then you have met the gap between hardness and toughness. The Mohs scale ranks gem and mineral hardness from 1 (talc) to 10 (diamond), but hardness is only one factor in durability 1.
So why did your stone chip? Hardness means resistance to scratching, not resistance to breaking. Even diamond, the hardest known natural mineral, is relatively brittle and can chip or fracture if it is not protected from shock 2. This article explains what else decides whether a stone survives everyday wear.
- The Mohs number measures scratch resistance, while toughness, the resistance to chipping and breaking, is a separate property 2.
- Stability covers chemicals, light, temperature and humidity, and no gem scores a "perfect 10" on hardness, toughness and stability together 1.
- How a stone is set and worn matters: jewellers use protective mountings for topaz or place it in low-wear pieces such as pendants and pins 3.
- Treatments add their own risks: any coatings on tanzanite are not permanent and can be scratched off in normal wear 6.
Why the Mohs scale is only one part of gemstone durability
Why look beyond one number? Because gemmologists, the specialists who study gems, define durability much more broadly. To them, it is a gem’s ability to withstand wear, heat, light, household chemicals, and low or high humidity 1. They judge it on three factors:
- Hardness: how well a stone resists scratches and abrasion (wearing away by rubbing).
- Toughness: how well it resists breaking and chipping.
- Stability: how well it resists chemicals and thermal shock, which is damage from a sudden change in temperature 2.
No gem scores a "perfect 10" on all three 1. A stone can do very well on one and poorly on another. That is how a high Mohs number and a chip can both be true of the same stone.
Why the steps on the Mohs scale are not equal
Why does it matter how the scale was built? Because it is easy to read 9 and 10 as close neighbours. Friedrich Mohs introduced the scale in 1822, ranking minerals by how easily one scratches another. The scale is not linear: corundum is 9, but diamond at 10 is many times harder 1.
GIA calls the Mohs scale deceptive for this reason, because the steps between minerals are not evenly spaced 3. The number tells you the order of scratch resistance. It does not tell you how much harder one stone is than another, and it says nothing about knocks.
Why toughness decides whether a stone chips
If hardness is about scratches, what decides chips? That is the job of toughness. Toughness is resistance to breaking and chipping, and it depends on how a gem’s atoms bond together 1.
Why does this surprise so many people? Because hardness and toughness are separate parts of durability, measured in different ways 2. These examples show how far apart they can be:
- Diamond is extremely hard but only somewhat tough. GIA calls toughness "the Achilles heel of diamond", meaning its weakest point 2.
- Emerald is 7.5 to 8 on the Mohs scale but has only fair to good toughness, so it needs more care in wear than ruby or sapphire 4.
- Jadeite is 6.5 to 7 and nephrite is 6 to 6.5, yet both types of jade have exceptional toughness and resist breaking and chipping very well 5.
- Tanzanite is 6 to 7, with fair to poor toughness and cleavage, "a tendency to break when struck" 6.
Compare the two ends of the scale. Jade needs some care to avoid scratches, yet it resists breaking and chipping very well 5. Diamond sits at 10, yet it will chip or fracture if it is not protected from shock 2.
Why cleavage creates hidden weak spots
Why can a very hard stone split cleanly? The answer is cleavage: flat directions inside a crystal along which it tends to split when struck. Diamond has four perfect cleavage directions, and cleavage planes are the main source of damage to diamonds 2.
Topaz shows the same pattern. It sits at 8 on the Mohs scale, but its toughness is rated poor because of its cleavage. A hard blow can split it, and extreme pressure or sharp temperature changes can break it 3.
Gem cutters work around this weakness. With topaz, they orient the cleavage about 15° to the table, the large flat facet on top of the stone, to reduce breakage 3.
Why heat, light and chemicals can harm a stone
Why should a ring care about perfume, cleaning products or a sunny windowsill? Because stability, the third part of durability, means resistance to chemicals, light, and changes in temperature or humidity 1. A stone can be hard and tough and still be damaged by everyday exposure.
- Heat: thermal shock can damage apatite, iolite, kunzite, opal and tanzanite. Opal can also crack or craze (form a web of fine cracks) in low humidity.
- Light: citrine, amethyst, kunzite and topaz can fade with long exposure to light.
- Chemicals: chlorine, perfume and makeup can damage pearls, and ammonia damages the polish on malachite, turquoise and coral 1.
- Acids: tanzanite is stable under normal wear, but hydrochloric and hydrofluoric acids attack it 6.
- A torch: jade is stable to light, but heat from a jeweller’s torch can harm it, and warm acids can affect it 5.
Why treatments add their own stability risks
Why do treatments matter here? Some gems are treated with coatings or fillings to improve how they look, and the treatment can be less durable than the stone. Coatings and fracture fillings are not permanent 1.
Fracture filling means filling cracks in a stone with oil, resin or a polymer so they show less. Most cut natural emeralds have filled fractures. Light and chemicals can change or break down these fillers, and steam can make oil or unhardened resin fillers "sweat out" 4.
Coatings behave in a similar way. Metallic-oxide coatings on topaz are not very durable, and abrasive cleaners or buffing wheels will remove them 3. Any coatings on tanzanite can be scratched off in normal wear 6. It helps to know how gemstone treatments should be disclosed before you buy.
Why shape and setting change real-life durability
Why can two stones of the same type wear so differently? Because the cut and the metal around a stone decide which parts are exposed. In diamonds, chips usually happen when the girdle, the thin outer edge where the top and bottom of a stone meet, or a point hits something hard 2.
Which diamond shapes have exposed points?
A stone's shape decides which parts of it can take a knock. Switch between six common shapes to see where the points and corners sit.
- Round
- A round has no exposed points. Chips in diamonds usually happen at the girdle or at a point, so the round's outline leaves less to catch.
- Oval
- An oval has no exposed points either. Its outline is smooth and rounded, so there are no corners to strike a hard surface.
- Cushion
- A cushion has corners, but they are rounded, so it has no exposed points. That sets it apart from shapes with sharp corners.
- Pear
- A pear has an exposed point. A very thin girdle also makes a stone prone to chipping, so the point deserves protection.
- Marquise
- A marquise has exposed points. A hard knock on a point is how many diamond chips start, and a very thin girdle adds to the risk.
- Princess
- A princess cut has square corners, and they are very prone to chipping because they lie close to a cleavage plane.
The pictures show loose stones. How a real diamond wears also depends on its girdle thickness, how it is set and how it is worn. Pictures: AI-generated illustrations, checked for accuracy. They are not photographs of a real piece.
- Pear and marquise shapes have exposed points, and very thin girdles are prone to chipping 1.
- Princess-cut corners are very prone to chipping because they lie close to a cleavage plane. Rounds, ovals and cushions with rounded corners do not have exposed points.
- Tension settings, where the band’s pressure holds the stone, already press on the girdle and leave its sides exposed.
- Bent or broken prongs, the small metal claws that hold a stone, are a very common cause of chipping.
- Inclusions, tiny crystals or breaks inside a stone, increase the risk when they sit near the girdle or points 2.
Settings can also protect a stone. Bezels (a metal rim around the stone), partial bezels or V-shaped prongs protect points and corners 1. You can compare styles in ring settings explained and see how shapes differ in choosing a diamond shape.
Why the way you wear a piece changes the risk
Why does the type of jewellery matter too? Because some pieces take far more knocks than others. To protect less tough gems, such as opal or tanzanite, consider earrings or necklaces, which are less likely to take blows 1.
Everyday habits matter too. With diamonds, countertops and diamonds worn next to each other are common causes of chips 2.
A less tough stone is not ruled out of jewellery. Jewellers use protective mountings for topaz or place it in low-wear pieces such as pendants and pins 3. Tanzanite can be worn in any kind of jewellery if it is protected from rough wear and hard blows 6.
The downsides of judging a stone by any single rating
Why not simply pick the hardest or toughest stone and stop worrying? Because no gem scores a "perfect 10" on hardness, toughness and stability together 1. Each part of durability can let you down in a different way:
- A very hard stone can still chip: diamond is only somewhat tough, and its hardness is uneven along different crystal directions 2.
- A very tough stone can still scratch: jade needs some care to avoid scratches 5.
- A hard stone can still fade: some yellow-to-brown topaz can fade with prolonged heat or sunlight 3.
GIA puts it simply: "There is no such thing as perfect toughness." Any gem will break if it is hit hard enough 2. Which trade-off you accept depends on how often you will wear the piece, what you do with your hands, and how much care you want to give it.
Why it pays to ask questions before you choose a stone
Why ask questions before you buy? Because the facts that matter are rarely the number on the label. These questions cover all three parts of durability:
- How tough is the stone, and does it have cleavage?
- Is it sensitive to heat, light, chemicals or humidity?
- Has it been treated, filled or coated, and how does that change its care?
- Does the shape have exposed points or a thin girdle?
- Will the setting protect points and edges, and are the prongs in good condition?
- Will it be worn in a ring, or in earrings or a necklace, which are less likely to take blows 1?
For stones often discussed for daily rings, see which gemstones suit everyday rings. Each stone’s own entry, such as tanzanite, topaz or opal, goes into more detail.
If you would like to talk through how a particular stone and setting might suit the way you live, Ralph Jacobs offers a free consultation. You can book online from any country at bookings.ralphjacobs.org.
Frequently asked questions
Yes. Diamond is extremely hard but only somewhat tough, with four perfect cleavage directions. Chips usually happen when the girdle or a point hits something hard, such as a countertop or another diamond worn next to it 2.