The Mohs Hardness Scale Opal Rating: What 5.5 to 6 Means

The Mohs hardness scale opal rating sits at roughly 5.5 to 6. That puts this flickering, rainbow-flashing gem in the middle of the ten-step scale: harder than an ordinary steel nail, about level with window glass, and noticeably softer than quartz. That middle position explains most of how opal behaves in a jewelry box, on a finger and on a crystal shelf. Below you will find how the scale works, where opal falls among other stones, what that means for rings and pendants, and the small habits that keep an opal looking as bright as the day you bought it.

What the Mohs Hardness Scale Opal Rating Means

“Mohs hardness” is a measure of scratch resistance. It tells you which materials can visibly scratch a stone and which ones it can scratch in turn. It is not a measure of weight, strength or how well a gem survives being dropped. When a reference chart lists the Mohs hardness scale opal figure as 5.5 to 6, it is making a simple comparative statement. Opal can scratch anything softer than about 5.5, and anything harder than about 6 can scratch opal.

In everyday terms, that rating means:

• Ordinary steel (around 4 to 4.5) should not scratch opal. A house key brushing past it is less of a threat than you might expect.
• Window glass (5.5) is a near match. Glass and opal can mark each other, depending on the exact piece.
• Quartz (7) will scratch opal. So will garnet, topaz, sapphire, ruby and diamond, which covers most of the gems people wear alongside it.

The rating is a range rather than a single number because natural materials vary. Two opals from different deposits, or even different parts of one rough stone, may not test identically. A Mohs hardness scale opal range of 5.5 to 6 is an honest way of saying “around the middle, toward the softer side of gemstones.”

How the Mohs Scale of Mineral Hardness Works

The Mohs scale of mineral hardness was introduced in 1812 by the German mineralogist Friedrich Mohs. The idea behind it is far older, though. Comparing stones by which one scratches the other was described by Theophrastus around 300 BC and by Pliny the Elder in the first century AD. Mohs gave the method a tidy structure: ten common reference minerals, ranked from softest to hardest.

The right-hand column shows why the scale needs careful reading. The Mohs mineral hardness scale is ordinal, which means it tells you the order of hardness, not the size of the gaps between steps. Quartz measures roughly twice as hard as apatite on an absolute instrument, and corundum about four times as hard as quartz. Diamond is in a league of its own. So the distance between opal and quartz is bigger than the one-point difference suggests. That is part of why quartz grit is so unkind to an opal surface, and why the Mohs hardness scale opal number should never be read as “almost as hard as quartz.”

To find a stone’s rating, you look for the hardest material it can scratch, or the softest material that can scratch it. For example, a mineral that apatite scratches but fluorite does not would sit between 4 and 5. A “scratch” here means a permanent mark visible to the naked eye, not a microscopic dent. If you want to see the full set of reference stones, our guide to the minerals on the Mohs hardness scale walks through each one. The article on the Mohs hardness scale test kit explains how geologists use them in the field.

The scale has limits. It works well for roughly identifying minerals, but it is not an accurate predictor of how a material will hold up under real wear. It can also mislead for rocks, which are mixtures of minerals. Granite, for instance, contains several minerals with their own ratings. Opal is a single gem material, so its rating is more meaningful than a rock’s would be, but the same caution about real-world wear still applies.

How Opal Compares With Other Gemstones

Read the Mohs hardness scale opal comparison like this. Anything above opal in the table below can scratch it, and anything below it generally cannot. The figures come from standard reference lists, given as ranges where materials vary.

A few comparisons come up often. Sapphire and ruby share the same rating of 9, matching corundum. Neither is harder than the other, and both are many times harder than opal. Our pieces on sapphire hardness and the ruby rating explain what a 9 means in practice. The glass rating article is a useful companion here, because glass is the material whose rating sits closest to the Mohs hardness scale opal band.

Turquoise and pearl are often grouped with opal as gems that need gentle care. Turquoise is generally listed in a similar mid-scale band, so the two are best treated alike. Pearl is softer still. If you are building a whole jewelry collection around soft stones, the broader gemstone hardness chart and our talc-to-diamond gems chart put dozens of stones side by side.

Hardness Versus Toughness: Why Opal Needs Gentle Handling

Hardness and toughness are easy to confuse. The Mohs mineral hardness scale only answers one question: what scratches what? It says nothing about how a stone responds to a sharp knock, pressure or a fall onto tile. That second quality is usually called toughness, and a stone can score well on one and poorly on the other. Diamond, at 10, still needs care, because a hard blow at the wrong angle can chip it.

For opal, the Mohs hardness scale opal number is only half the story. Jewelers and collectors generally treat opal as a gem that should be protected from knocks and rough handling, not just from scratches. A bump against a door frame might not leave a scratch, yet it can still damage a stone that is not well protected by its setting. That is why the advice for opal focuses as much on settings and situations as on the number itself.

Think of it this way: the hardness rating tells you what not to store opal with, and toughness tells you what not to do while wearing it.

Wearing Opal: Rings, Pendants and Earrings

Because the Mohs hardness scale opal rating sits in the middle of the scale, the type of jewelry matters more than it does for harder gems.

Rings

Rings take the most punishment of any jewelry. They brush against desks, car doors, gardening gloves and kitchen counters, and they pick up grit along the way. An opal ring can absolutely be worn, but it does best with a protective setting. A bezel, where a metal rim wraps the edge of the stone, is a good choice, and a slightly recessed stone is better than one that sits proud of the band. Take it off for cleaning, gardening, the gym and anything involving sand or stone.

Pendants and earrings

These are the easy options. A pendant hangs away from most knocks, and earrings rarely touch anything harder than a hairbrush. If you love opal and want to wear it every day without much worry, these are the friendliest forms.

A worked example

Imagine you wear an opal ring daily alongside a band set with small garnets, and in the evening both go into the same dish with an amethyst bracelet. On the Mohs scale of mineral hardness, garnet (7 to 7.5) and amethyst, a quartz (7), both outrank opal. Each evening the pieces jostle together, and over months the opal’s polished dome can develop a faint haze of fine scratches that dulls its play of colour.

The fix costs nothing. Give the opal its own soft pouch or a lined compartment, and wipe it with a soft cloth before putting it away so any grit comes off rather than being ground in. The Mohs hardness scale opal ranking does not change, but the stone’s daily exposure to harder materials drops to almost nothing.

Grit deserves a special mention. Household dust and outdoor dirt can include tiny grains of quartz sand. Even an occasional wipe with a dusty cloth can drag those grains across the surface. Always clean opal with a fresh, clean cloth.

How to Clean and Store Opal

A simple routine, built around the Mohs hardness scale opal rating, keeps the stone looking its best. These steps suit solid, untreated opal. If your piece is assembled from layers, such as a thin slice of opal backed with another material, ask a jeweler before getting it wet, and never soak it.

1. Dust first. Use a soft brush or a clean microfibre cloth to lift away loose grit before any wiping, so you do not rub particles into the surface.
2. Wash gently if needed. A little lukewarm water with a drop of mild soap, applied with a soft cloth, is plenty for solid opal. Rinse and pat dry.
3. Skip harsh methods. Avoid ultrasonic cleaners, steam cleaners, bleach and household chemical cleaners. Avoid abrasive pastes or polishing compounds too.
4. Avoid salt cleansing. Burying crystals in salt is a popular energy-cleansing ritual, but salt crystals are gritty. They are a poor match for a stone that sits in the middle of the Mohs hardness scale. Opal is also not a good candidate for crystal elixirs or long soaks.
5. Store it alone. Use a fabric pouch or a lined compartment, away from quartz, garnet, sapphire and anything else harder than 6.
6. Never scratch-test it. Testing a finished gem against glass or a steel point leaves a permanent mark. Leave hardness testing to rough, inexpensive specimens.

If you follow a cleansing practice for energetic reasons, gentler methods suit opal well. Many people like moonlight, sound from a bell or singing bowl, smoke from a herb bundle held at a safe distance, or simply setting the stone on a clean cloth with a quiet intention. None of these touch the surface, so the Mohs hardness scale opal rating never comes into play.

Opal’s Meaning in Crystal Traditions

Opal is recognised as one of October’s birthstones, which you can explore alongside the other months in our zodiac birthstones guide. In crystal traditions, it is often associated with creativity, inspiration and emotional expression. Its shifting flashes of colour are read as a symbol of change and possibility. Many people keep opal nearby during creative work, believing it helps them feel more open to ideas. Others use it in meditation as a focus for reflecting on their feelings.

Its folklore is mixed and lively. Over the centuries, opal has been described both as a stone of good fortune and hope and, in some periods and places, as unlucky. That contradiction is part of its charm. It reflects how strongly people have always responded to a gem that seems to hold moving light inside it. Our editorial approach explains how we present beliefs like these: as traditions people find meaningful, not as proven effects.

A few ways people use opal day to day, each shaped by the Mohs hardness scale opal rating:

• Carrying. A tumbled opal (one smoothed and polished by rolling in a barrel) can be carried in a small pouch, never loose in a pocket with keys, coins and other stones.
• Meditation. Holding a raw (uncut, natural) or polished piece and watching its colours shift is a simple focusing practice many people find calming.
• Placement at home. A desk or creative workspace is a favourite spot, set on a soft cloth or in a small dish of its own.
• Jewelry. Pendants and earrings let the stone stay close with less risk than a ring.

If you would like a piece of your own, you can browse opal at Spirit Magicka, the journal’s own crystal shop. You may also come across opalite, which looks milky with a soft blue-orange glow. Opalite is usually a manufactured glass rather than natural opal. It is loved in its own right, but it is a different material, and it sits around glass hardness, close to the Mohs hardness scale opal range. Treat it with the same gentle care.

For more stones with a similar mid-scale softness, our Earth treasures collection gathers the library’s articles on minerals outside the quartz and moonstone families. The full crystal library holds the rest.

Common Mistakes With Opal and the Mohs Scale

Most opal damage comes from a handful of habits, not from bad luck. These are the ones worth avoiding.

• Reading the number as “fairly hard.” A 5.5 to 6 sounds like the middle of the road. But because the scale is ordinal, quartz at 7 is much harder than opal, not slightly harder.
• Storing opal with harder crystals. A shelf display of amethyst points, citrine and garnet tumbles is lovely. Just keep the opal on its own cloth rather than in the same bowl. The quartz family stones in particular all rate 7.
• Assuming hardness equals durability. The Mohs hardness scale opal figure says nothing about knocks. A well-protected setting matters as much as the rating.
• Wearing an opal ring for rough jobs. Gardening, lifting, cleaning with gritty products and beach days are the classic culprits.
• Testing hardness on a finished stone. Even a curious, careful scratch leaves a permanent mark on a polished surface.
• Using a dirty cloth. Wiping with a dusty cloth can drag hard grit across the stone.
• Assuming an imitation is tougher. A lab-made or glass alternative is only as hard as its own material, so ask what a piece is made of before relying on it.

Putting Opal’s Hardness Into Perspective

The Mohs hardness scale opal rating is not a warning label. It is a guide to living well with a beautiful stone. Opal sits comfortably above steel, roughly beside glass and below most popular gems. Once you know that, caring for it comes down to three habits: keep it away from harder stones, keep it out of situations where it might take a knock, and clean it with soft, clean materials.

If you are curious how opal compares with the extremes, our pieces on diamond’s 10 and emerald’s hardness and toughness show how the same scale applies at the harder end. Our explainer on how crystals form gives some background on why minerals end up so different from one another.

Whatever draws you to opal, whether its October birthstone tradition, its symbolism of creativity, or simply the way its colours move when you tilt it, a little knowledge of the Mohs hardness scale opal rating lets you enjoy it for years. And when you are ready to add to your collection, Spirit Magicka is the journal’s own shop for crystals and minerals.