The Mohs Scale Explained: Why Your Crystal Scratches Glass (Or Does Not)

The Mohs Scale Explained: Why Your Crystal Scratches Glass (Or Doesn't)

Someone once told me they knew their crystal was real because it could scratch glass. I asked which crystal. They said fluorite. Fluorite has a Mohs hardness of 4. Glass is around 5.5. Their crystal couldn't scratch glass — it was being scratched by the glass. They'd been testing it wrong for months.

The Mohs scale is probably the single most useful thing you can learn as a casual crystal collector, and it's not complicated. Here's what it actually means, how to use it, and the common mistakes that lead people astray.

What the Mohs Scale Measures

Developed by German mineralogist Friedrich Mohs in 1812, the Mohs hardness scale ranks minerals from 1 (softest) to 10 (hardest) based on their ability to scratch each other. A mineral with a higher number can scratch any mineral with a lower number. That's the entire system.

It's a relative scale, not a linear one. The jump from 9 (corundum) to 10 (diamond) is actually a bigger gap in absolute hardness than the entire range from 1 to 9 combined. But for practical identification purposes, the relative rankings are what matter.

The ten reference minerals:

• 1 - Talc: Can be scratched with a fingernail
• 2 - Gypsum: Can be scratched with a fingernail (barely)
• 3 - Calcite: Can be scratched with a copper coin
• 4 - Fluorite: Can be scratched with a steel knife
• 5 - Apatite: Can be scratched with a steel knife with effort
• 6 - Orthoclase: Can scratch glass, can be scratched by a steel file
• 7 - Quartz: Easily scratches glass
• 8 - Topaz: Scratches quartz
• 9 - Corundum: Scratches topaz (ruby and sapphire are corundum)
• 10 - Diamond: Scratches everything, nothing scratches it

Common Crystals and Where They Fall

Most crystals you'll encounter in shops fall between 2 and 8 on the scale. Knowing where your pieces sit helps with everything from storage decisions to authenticity checks.

Soft (Mohs 1-3) — Handle With Care

• Selenite (2): Can be scratched with a fingernail. Also dissolves in water. The most fragile common crystal.
• Malachite (3.5-4): Softer than you'd expect from how solid it looks
• Calcite (3): Effervesces in weak acid (vinegar) — a fun identification test
• Pearl (2.5-3): Why perfume and hairspray damage pearl jewelry

Medium (Mohs 4-6) — Normal Handling

• Fluorite (4): Will get scratched if stored with quartz. Keep it separate.
• Lapis lazuli (5-5.5): Durable enough for daily-wear jewelry but don't store it rough-and-tumble with harder stones
• Apatite (5): The "what mineral?" of the crystal world — fairly common but nobody talks about it
• Labradorite (6-6.5): The reason it's safe for everyday pendants

Hard (Mohs 7+) — Tough Customers

• Quartz (7): Amethyst, citrine, rose quartz, smoky quartz, clear quartz — all the same hardness. Will scratch glass easily.
• Topaz (8): The surprise hard one. Feels like it should be softer but isn't.
• Corundum (9): Ruby and sapphire. Second hardest natural material. Practically indestructible for daily wear.

Practical Scratch Tests You Can Do at Home

These tests won't tell you exactly what a mineral is, but they can narrow it down quickly. All you need are common household items.

The Fingernail Test (Mohs ~2.5)

Can you scratch the mineral with your fingernail? If yes, it's 1-2 on the Mohs scale. You're looking at something like talc, selenite, or gypsum. Very few "display crystals" fall this low, so if your specimen is scratchable by fingernail, it's either selenite or something unusual.

The Copper Coin Test (Mohs ~3)

A pre-1982 US penny (solid copper) or equivalent copper coin can scratch minerals at Mohs 3 and below. Calcite (3) is right on the edge — a coin will barely scratch it.

The Steel Knife Test (Mohs ~5.5)

A steel pocketknife or kitchen knife blade will scratch minerals up to about Mohs 5. If your knife can't scratch it, the mineral is 5.5 or harder. This is a useful dividing line — it separates the "needs careful storage" group from the "fairly durable" group.

The Glass Scratch Test (Mohs ~5.5)

This is the one most people know, but they often get it backwards. Glass is about 5.5 on the Mohs scale. If a mineral scratches glass, it's harder than 5.5 (quartz, topaz, etc.). If glass scratches the mineral, the mineral is softer than 5.5 (fluorite, calcite, etc.).

To test properly: find a piece of scrap glass (an old jar). Hold the mineral firmly and press its point or edge firmly across the glass surface. You should feel resistance and see a visible scratch line. Don't use gentle pressure — you need firm, deliberate contact.

Why Hardness Matters for Collectors

Storage

Harder minerals scratch softer ones. If you throw all your crystals in a box together, quartz (7) will scratch fluorite (4) and fluorite will scratch calcite (3). The dust in your storage container is often quartz particles (silica dust, Mohs 7), which means even leaving soft minerals exposed on a shelf leads to gradual surface damage.

Solution: wrap soft minerals individually, store hard and soft separately, and don't let specimens rattle against each other.

Wearability

For jewelry that gets daily wear (rings especially), you want Mohs 7 or above. Ring surfaces get knocked against doorframes, countertops, and steering wheels constantly. A fluorite ring would be destroyed within a week of daily wear. A quartz or corundum ring will last decades.

For occasional-wear pendants and earrings, Mohs 5 and above is usually fine — they don't take the abuse that rings do.

Cleaning

Harder minerals can handle more vigorous cleaning. You can scrub quartz with a stiff brush without worry. Do that to selenite and you'll physically remove material. Ultrasonic cleaners are safe for hard stones (7+) but can crack or degrade softer ones.

Hardness vs. Toughness: They're Not the Same

This distinction matters and confuses a lot of people. Hardness is scratch resistance. Toughness is break resistance. They don't correlate.

• Diamond (10 hardness, fair toughness): Can't be scratched but can be shattered with a hammer. The "hit it with a hammer" test in heist movies is actually accurate — diamonds are brittle despite being the hardest material.
• Jade (6-7 hardness, exceptional toughness): Not particularly hard, but incredibly resistant to breaking. This is why jade has been carved into intricate shapes for thousands of years — it doesn't shatter.
• Topaz (8 hardness, perfect cleavage): Very hard, but has a plane along which it splits cleanly with a single impact. Drop a topaz on a hard floor at the wrong angle and it can split in two.

For everyday practical purposes: hardness tells you what you can store it with and how to clean it. Toughness tells you whether you should worry about dropping it.

One Last Thing

The scratch test is one tool among many. It can narrow down possibilities but rarely identifies a mineral by itself. A stone that scratches glass could be quartz, topaz, corundum, tourmaline, or several other minerals. You need additional clues — color, crystal habit, streak, specific gravity, and sometimes refractive index — to make a confident identification.

But knowing the Mohs scale and where your common crystals fall on it will immediately level up your collecting. You'll stop storing incompatible stones together, you'll know which pieces are safe for daily-wear jewelry, and you'll be able to spot obvious fakes (if a "diamond" can be scratched by a steel knife, it's not diamond). Three practical wins from one simple scale.