Understanding Gemstone Hardness on the Mohs Scale

What the Mohs Scale Actually Measures

Most people meet the Mohs scale as a row of numbers on a jeweller's website and read it as a league table of quality. It isn't. The scale, devised by the German mineralogist Friedrich Mohs in 1812, simply ranks ten reference minerals by their ability to scratch one another: talc at 1, gypsum at 2, calcite at 3, fluorite at 4, apatite at 5, feldspar at 6, quartz at 7, topaz at 8, corundum at 9 and diamond at 10.

The important word is scratch. A gemstone can only be scratched by a material of equal or greater hardness, which makes the scale a practical guide to what you can safely let a stone touch: keys, coins, cutlery, sand, a granite worktop, another piece of jewellery.

It also helps to know that the numbers are not evenly spaced. The jump from 9 to 10 is far larger than the jump from 1 to 2. Diamond is roughly four times harder than corundum in absolute terms, even though the two sit only one step apart.

Where Onyx Sits on the Scale

True onyx is a variety of chalcedony, which is microcrystalline quartz, so it lands at 6.5 to 7. That is respectably hard. It sits above window glass at around 5.5, above steel, and above most household dust. Above it lie topaz at 8, corundum at 9 — ruby and sapphire — and diamond at 10.

There is a catch, though, and it catches people out. Not everything sold as onyx is quartz. Mexican onyx and onyx marble, both common in ornaments and occasionally in jewellery, are banded calcite with a hardness of about 3. That is softer than a copper coin and will scratch with very little encouragement. Black onyx is also frequently dyed, and dye can sit in surface-reaching fractures where solvents, alcohol-based perfume and harsh cleaners can lift it out.

Hardness Is Not Toughness

These two properties get muddled constantly. Hardness is resistance to scratching. Toughness is resistance to breaking, chipping and cracking. Jade is only 6 to 6.5 yet is famously tough; diamond is the hardest substance known and will still cleave along a plane if struck at the wrong angle.

Onyx sits in the middle ground. It takes a lovely mirror polish and wears well in a well-made setting, but it is brittle, and thin or unprotected edges can chip rather than scratch. A bezel or rub-over setting, which wraps metal around the girdle of the stone, will protect an onyx far better than claws. Hardness alone will not save a stone from a knock, a drop, or a sudden change in temperature.

Everyday Scratch Risks

Think about what your jewellery actually meets during a normal week.

• Loose in a handbag or pocket with keys and coins. A diamond ring will scratch an onyx cabochon, and even silver or gold will scuff it over time.
• Stacked rings on one hand. A diamond eternity band worn beside an onyx ring will slowly win the argument.
• Sand and grit. Beach sand is largely quartz at 7, the same region as onyx, so it abrades rather than cuts.
• Ceramic, porcelain and granite surfaces, which hover around 6 to 7 and can leave dull scuffs.
• Gardening and gym work, where the real risk is impact rather than scratching.

It also works in reverse. At 6.5 to 7, onyx is hard enough to scratch softer gemstones it shares a jewellery box with: pearls (2.5 to 4.5), opals (5.5 to 6.5), turquoise (5 to 6), amber and coral. An onyx bead bracelet rubbing against a pearl strand will dull the pearls within weeks.

Pairing Onyx with Harder Gemstones

Mixed-stone jewellery is beautiful and entirely practical, provided the design respects the differences.

• Separate at rest. Individual pouches or a compartmented box. This single habit prevents most damage.
• Think about placement. If a piece mixes onyx with sapphire or diamond, keep the harder stones on the outer edges and the onyx in a protected centre, or use metal spacers between them.
• Consider the metal, not just the stone. 9ct gold is harder wearing than 18ct or 22ct, which are softer and mark more readily. Platinum is dense and durable but still shows wear. Settings matter more than the alloy, though — a rub-over setting is worth more than a fancy hallmark.
• Never wear an onyx bracelet on the same wrist as a diamond tennis bracelet. The constant contact will dull the onyx and stretch the metal.

Caring for Onyx and Mixed-Stone Pieces

Onyx is easy to look after once you know the routine.

• Put jewellery on last and take it off first, so perfume, hairspray and moisturiser never settle on the surface.
• Clean with lukewarm water, a drop of mild washing-up liquid and a very soft brush, then rinse and dry with a soft cloth. Avoid ultrasonic and steam cleaners on dyed or treated onyx, and skip ammonia, bleach and acidic cleaners altogether.
• Keep it away from prolonged direct sun, which can fade dyes, and avoid sudden temperature changes.
• Wipe pieces after wearing and dry them thoroughly before storing.
• Have settings checked once a year. A loose stone chips far more easily than a secure one.

If an onyx does pick up fine scratches or a dull patch, all is not lost. Chalcedony takes an excellent polish, and a lapidary can re-polish a cabochon or bead so it looks new again, usually far more cheaply than replacing the stone.