Jewellery Care by Gemstone Hardness: The Mohs Scale Explained
The safest way to clean a gemstone is decided by more than how hard it is. Hardness tells you how easily a stone scratches, and the Mohs scale ranks that from talc at 1 to diamond at 10, a system the USGS traces back to the Austrian mineralogist F. Mohs in 1822. But hardness says nothing about whether a stone will survive an ultrasonic bath. Topaz sits at 8, harder than most stones in your box, and GIA still says to keep it out of steam and ultrasonic cleaners. This guide covers both halves: what scratches what, and what actually goes in the machine.
What the Mohs Scale Measures, and What It Misses
Mohs is a scratch test and nothing more. A stone higher on the scale will scratch anything below it, which is why storing a loose diamond next to a gold chain marks the gold rather than the stone. The steps are not evenly spaced either. The gap between corundum at 9 and diamond at 10 is far wider than the gap between 8 and 9, so "one point apart" means very different things at different ends of the scale.
The number that matters most for daily wear is 7. The USGS makes the reason plain: much household dust is silica, which is quartz, and quartz is 7. A stone softer than the dust in the air will slowly go dull from nothing more than being worn. That is the real argument for sapphire or diamond in a ring you never take off, and the real reason an opal ring belongs on your finger for dinner rather than for gardening.
What Mohs does not measure is toughness, which is resistance to chipping and breaking. The two come apart badly in a few cases. Tanzanite is a respectable 6 to 7 for scratching, but GIA notes it has fair to poor toughness and a property called cleavage, which is a tendency to break when struck. A knock on a doorframe, not a scratch, is what ends most tanzanite rings.
Hardness by Stone
Every figure below is GIA's own published value for that gem. Where a range is given, it reflects real variation in the material, not disagreement between sources.
| Stone | Mohs hardness | Everyday ring? |
|---|---|---|
| Diamond | 10 | Yes |
| Moissanite | 9.25 | Yes |
| Ruby | 9 | Yes |
| Sapphire | 9 | Yes |
| Topaz | 8 | With care |
| Emerald | 7.5 to 8 | Occasional wear |
| Aquamarine | 7.5 to 8 | With care |
| Amethyst | 7 | With care |
| Garnet | 6.5 to 7.5 | With care |
| Tanzanite | 6 to 7 | Occasional wear |
| Opal | 5 to 6.5 | Occasional wear |
| Turquoise | 5 to 6 | Occasional wear |
| Pearl | 2.5 | No |
Pearl at 2.5 is the one that surprises people. That is softer than a fingernail is hard against it in practical terms, and it is why a strand worn against a wool coat collar all winter comes back with the nacre worn thin at the front. We see it every year in the shop, always on the same three or four pearls that sat at the throat.
The Ultrasonic Question, Answered Stone by Stone
An ultrasonic cleaner works by driving microscopic bubbles through the solution at high speed. That energy lifts grime out of a setting beautifully. It also finds every existing fracture in a stone and pushes on it. Steam adds heat to the same problem.
Never: pearl, opal, turquoise, tanzanite and topaz. GIA is unambiguous on each: pearls should never be cleaned in an ultrasonic or steam cleaner; the only safe way to clean opal is with warm, soapy water; turquoise should never be cleaned with steam or ultrasonic cleaners; and ultrasonic and steam cleaners are never recommended for tanzanite.
Risky: emerald. Because some estimates state that 90 percent or more of emeralds are fracture-filled, and because ultrasonic vibrations can weaken an already-fractured stone while hot steam can cause oil or unhardened resin to sweat out of the fractures, GIA calls cleaning emerald this way risky. Warm soapy water and a soft brush is the method.
Usually fine, with conditions: ruby and sapphire are safe for untreated, heat-treated and lattice-diffusion-treated stones. Aquamarine is usually safe unless the stone has liquid inclusions or fractures. Amethyst and garnet are usually safe except where a stone is fracture-filled or dyed, and neither should be steamed.
One caution applies to every stone, diamond included. GIA notes that ultrasonic cleaners and steam cleaners can loosen gemstones in their settings. The stone survives; the setting gives up. If you run an ultrasonic at home, check your prongs afterwards, or read our guide to the warning signs of a loose stone first.
Why Emerald and Opal Behave the Way They Do
Emerald's problem is that most emeralds on the market have surface-reaching fractures that have been filled with oil, resin or polymer to improve clarity. GIA warns that heat can damage emeralds, especially by extending existing fractures, and that light and chemicals can make those fillers alter in appearance or deteriorate. The stone itself is a perfectly respectable 7.5 to 8. It is the treatment that needs protecting.
Opal fails differently. GIA reports that heat from intense light can cause fracture lines called crazing, and that high heat or sudden temperature change can fracture it outright. That rules out a hot ultrasonic tank, a steamer, and leaving an opal ring on a sunny windowsill. Our page on cleaning pearls and opals goes through the gentle method in detail.
The Home Method That Works for Almost Everything
Warm water, a drop of plain dish soap, a soft toothbrush, and a lint-free cloth. It is safe for every stone in the table above, including the ones that cannot go near a machine. Brush gently behind the setting where skin oil and lotion collect, rinse in clean warm water, and dry fully before putting the piece away.
Two exceptions to remember. Pearls get wiped, never soaked: GIA advises to wipe cultured pearls with a very soft, clean cloth after each wearing, and warns they are damaged by many chemicals and all acids, including hair spray, perfume, cosmetics and perspiration. If a strand is knotted on silk, the string must be completely dry before you wear it again. Opals get the same warm soapy water but no soaking and no temperature swings.
Storing Stones So They Stop Scratching Each Other
Most of the damage we repair did not happen during wear. It happened in a drawer, where a diamond stud spent two years sitting against a pearl pendant. Separate compartments, individual soft pouches, or a lined box with dividers solves it completely. Softer stones go furthest from anything hard. Our guide to storing jewellery properly covers the practical setups.
Pieces Chosen for Everyday Wear
These are current pieces from our Coquitlam shop, weighted toward stones that hold up to daily contact.
Key Takeaways
- Mohs measures scratching only. Toughness, treatment and setting decide whether a stone survives a cleaner.
- Aim for 7 or above in a ring worn daily, because household dust is largely quartz at 7.
- Keep pearl, opal, turquoise, tanzanite and topaz out of ultrasonic and steam cleaners, per GIA, regardless of hardness.
- Pearl is 2.5 on GIA's scale, the softest thing most people own, and needs wiping after every wear.
- Warm water, mild soap and a soft brush is safe for everything in this guide.
Frequently Asked Questions
Can I put my engagement ring in an ultrasonic cleaner?
If the centre stone is a diamond, sapphire, or ruby and it has not been fracture-filled, an ultrasonic is generally safe for the stone. The setting is the real risk. GIA warns that ultrasonic and steam cleaners can loosen gemstones in their settings, so a ring with worn prongs can come out of the tank with a stone sitting loose or missing entirely. If your ring has an emerald, opal, pearl, turquoise, tanzanite or topaz, skip the machine altogether and use warm soapy water instead. When in doubt, bring the ring in and let someone check the prongs under magnification before any cleaning, which takes a few minutes and costs nothing.
What is the softest stone I can safely wear in a ring every day?
The practical floor is around 7 on the Mohs scale, because household dust contains quartz particles at hardness 7 and anything softer will gradually abrade just from normal handling. That puts amethyst, garnet and the harder tourmalines at the edge of acceptable for daily wear, and it puts opal, turquoise, pearl and tanzanite firmly in the occasional-wear category. Hardness is only half the answer though. Tanzanite scores 6 to 7 but has cleavage, meaning it can split when struck, so it fails on toughness even where hardness looks borderline acceptable. A protective bezel setting buys a softer stone real extra life by shielding the edges from knocks.
Why did my pearls go dull even though I looked after them?
Pearls are 2.5 on the Mohs scale, softer than almost anything else you own, so they abrade against clothing, skin and any harder jewellery stored beside them. GIA also notes that pearls are damaged by many chemicals and all acids, which includes hair spray, perfume, cosmetics and ordinary perspiration. The usual pattern we see is wear concentrated on the few pearls that sit at the front of the throat, where a coat collar rubs them. Put pearls on last, after perfume and hair products have dried, wipe them with a soft cloth after every wear, and store them separately from everything else rather than in a shared compartment.
Is a harder stone always the better choice?
Not on its own, because hardness and toughness are separate properties and a stone can be excellent at one and poor at the other. Diamond is a 10 and still has cleavage directions along which a sharp blow can chip it, which is why diamond rings do occasionally arrive at the bench with a nicked girdle. Emerald sits at a respectable 7.5 to 8 but most emeralds contain filled fractures that make heat and vibration genuinely dangerous to them. The better question is how the stone will actually be worn, and whether the setting protects the vulnerable edges. A well-designed bezel around a softer stone often outlasts a high prong setting around a harder one.
Sources
- USGS, "Natural Gemstones: Mineral Gemstones"
- GIA, "Pearl Care and Cleaning Guide"
- GIA, "Opal Care and Cleaning Guide"
- GIA, "Emerald Care and Cleaning Guide"
- GIA, "Topaz Care and Cleaning Guide"
- GIA, "Tanzanite Care and Cleaning Guide"
- GIA, "Turquoise Care and Cleaning Guide"
- GIA, "Diamond Care and Cleaning Guide"
Hardness values retrieved from GIA's published gem care guides, August 2026. If something here is out of date by the time you're reading it, let us know, we keep our guides current as the trade changes.
Visit Vanhess
We clean and inspect jewellery on-site at our Coquitlam workshop, and we will tell you honestly which of your pieces should never see an ultrasonic tank. Bring in anything you are unsure about, see our full jewellery care guide, or call us at (604) 653-6449.
