Omikuji Tools Life & Calc

Mohs Hardness Chart

Hardness for gems and minerals, with cleavage and toughness, and everyday things to compare against.

✨ What you can do

  • See hardness together with cleavage and toughness for gems, minerals and everyday objects
  • Filter by group and search by name
  • Pick two materials and find out which scratches which
  • Spot the hard-but-fragile stones at a glance: topaz, tanzanite, fluorite

🪄 Good for

Choosing a ring you will wear daily

Hardness 7 or above with no cleavage stands up to everyday wear.

Caring for what you own

Stones with cleavage, plus opal and pearl, should avoid knocks and ultrasonic cleaners.

Studying mineralogy

The ten reference minerals sit next to the fingernail, coin and glass benchmarks.

Narrowing down an unknown stone

What it does and does not scratch brackets its hardness - though never test a real gem.

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❓ FAQ

What is Mohs hardness?

A scale of scratch resistance defined by the German mineralogist Friedrich Mohs in 1812, built from ten reference minerals: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum and diamond. It is an ordering, not a ratio - a 10 is not twice as hard as a 5.

Is diamond unbreakable?

No. Diamond wins on scratching, but it has cleavage planes that a sharp blow can split - the very property that was used to cut diamonds before modern sawing. Treat it with the same care as any other stone.

Which stones suit a ring?

Hardness 7 or more, no cleavage, and good toughness: ruby and sapphire (9), chrysoberyl (8.5), spinel (8), quartz (7). Opal, tanzanite, moonstone and fluorite are happier in a pendant or earrings.

Why is emerald chippy at hardness 7.5-8?

Emerald normally contains many internal fractures and inclusions, and those are where it fails. Hardness describes the surface's resistance to scratching, which is a different property. Avoid ultrasonic cleaners and knocks.

Can I test hardness myself?

A scratch test always leaves a mark on something, so never do it on a valuable stone. With mineral specimens, work from softer to harder and test on an inconspicuous face. For gem identification, use a laboratory.

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Which one gets scratched?

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Materials within half a point of each other can both end up scratched.

Group

10

Diamond

Perfect cleavage Toughness: Good Reference mineral 10

Cleavage: Perfect in four directions (octahedral)

The hardest mineral, yet vulnerable to a blow along its cleavage - the same property used to cut it.

9

Corundum (ruby, sapphire)

No cleavage Toughness: Exceptional Reference mineral 9

Cleavage: None (parting may occur)

Red is ruby, every other colour is sapphire - the same mineral. Hard and tough, so it wears well.

8.5

Chrysoberyl (alexandrite)

Poor cleavage Toughness: Exceptional

Cleavage: Poor

Both hard and tough. Alexandrite changes colour with the light source.

8

Topaz

Perfect cleavage Toughness: Fragile Reference mineral 8

Cleavage: Perfect in one direction

Hard yet splits along its cleavage - the classic example of hardness not meaning durability.

8

Spinel

No cleavage Toughness: Good

Cleavage: None

Long mistaken for ruby; no cleavage, so it holds up well.

7.5〜8

Beryl (emerald, aquamarine)

Poor cleavage Toughness: Fragile

Cleavage: Poor

Emerald is riddled with inclusions, so it chips more easily than its hardness suggests - never clean it ultrasonically.

7〜7.5

Tourmaline

No cleavage Toughness: Fair

Cleavage: None

One of the widest colour ranges of any gem; becomes electrically charged when heated.

7

Quartz

No cleavage Toughness: Good Reference mineral 7

Cleavage: None (conchoidal fracture)

Household dust is largely quartz - which is why gems softer than 7 dull over time.

6.5〜7.5

Garnet

No cleavage Toughness: Fair

Cleavage: None

A group of minerals - green and orange varieties exist alongside the familiar red.

7

Rock crystal, amethyst, citrine

No cleavage Toughness: Good

Cleavage: None

All quartz; the colours come from trace elements.

7

Household dust

No cleavage Toughness: Fair

Cleavage: —

Mostly quartz at hardness 7 - anything softer wears down simply by being worn.

6.5〜7

Jadeite

No cleavage Toughness: Exceptional

Cleavage: None (interlocking fibres)

Softer than quartz yet among the toughest of all gems, thanks to interlocking fibres - which is why it was worked into tools and ornaments.

6.5〜7

Peridot

Poor cleavage Toughness: Fragile

Cleavage: Poor

Sensitive to heat and thermal shock. The August birthstone.

6〜7

Tanzanite

Perfect cleavage Toughness: Fragile

Cleavage: Perfect in one direction

Far more fragile than its hardness implies - better in a pendant than a ring.

6.5

Steel file

No cleavage Toughness: Good

Cleavage: —

Cannot scratch quartz (7).

6〜6.5

Moonstone (feldspar)

Perfect cleavage Toughness: Fragile

Cleavage: Perfect in two directions

Two directions of perfect cleavage make it chip-prone; set it where it will not be knocked.

6

Orthoclase

Perfect cleavage Toughness: Fair Reference mineral 6

Cleavage: Perfect in two directions

A feldspar; moonstone belongs to this group.

5.5〜6.5

Opal

No cleavage Toughness: Fragile

Cleavage: None

Contains water; drying or thermal shock can craze it.

5〜6

Turquoise

No cleavage Toughness: Fair

Cleavage: None

Porous - cosmetics and skin oils can discolour it.

5.5

Window glass

No cleavage Toughness: Fragile

Cleavage: —

The classic scratch test - but it damages the stone too, so never try it on a gem.

5.5

Steel knife blade

No cleavage Toughness: Good

Cleavage: —

Cannot scratch orthoclase (6).

5〜5.5

Lapis lazuli

No cleavage Toughness: Fair

Cleavage: None (it is a rock, not a single mineral)

A rock of several minerals; once ground into the pigment ultramarine.

5〜5.5

Obsidian

No cleavage Toughness: Fragile

Cleavage: None (razor-sharp conchoidal fracture)

Natural glass; its edges are sharp enough that it was knapped into blades.

5

Apatite

Poor cleavage Toughness: Fragile Reference mineral 5

Cleavage: Poor

The same mineral family as tooth enamel and bone.

4〜4.5

Iron nail

No cleavage Toughness: Good

Cleavage: —

Will not scratch apatite (5).

4

Fluorite

Perfect cleavage Toughness: Fragile Reference mineral 4

Cleavage: Perfect in four directions (octahedral)

Beautifully coloured but cleaves easily - a poor choice for rings.

2.5〜4.5

Pearl

No cleavage Toughness: Fair

Cleavage: None

Very soft and sensitive to cosmetics, perfume and acids - put pearls on last and take them off first.

3〜4

Coral

No cleavage Toughness: Fair

Cleavage: None

Calcium carbonate, so acids attack it.

3.5

Copper coin

No cleavage Toughness: Fair

Cleavage: —

Scratches calcite (3) but not fluorite (4).

3

Calcite

Perfect cleavage Toughness: Fragile Reference mineral 3

Cleavage: Perfect in three directions (rhombohedral)

Scratched by a copper coin; always breaks into rhombs.

2.5

Fingernail

No cleavage Toughness: Fair

Cleavage: —

If your nail scratches it, the hardness is 2.5 or below.

2〜2.5

Amber

No cleavage Toughness: Fragile

Cleavage: None

Fossilised resin - very light and soft, and damaged by alcohol or heat.

2

Gypsum

Perfect cleavage Toughness: Fragile Reference mineral 2

Cleavage: Perfect in one direction

Just scratchable with a fingernail; used in plasterboard.

1

Talc

Perfect cleavage Toughness: Fragile Reference mineral 1

Cleavage: Perfect in one direction; peels into sheets

Scratched by a fingernail; the source of talcum powder.

Hardness is not durability

  • Mohs hardness ranks resistance to scratching. It is an ordering built by seeing which of two minerals marks the other, not a measured quantity - the steps are not equal, and the gap between 9 and 10 is famously larger than the one between 1 and 9.
  • Hard does not mean unbreakable. Diamond is the hardest mineral yet splits along its cleavage under a sharp blow, while jadeite - softer than quartz - is so tough that it was worked into tools for millennia.
  • The reason worn gems go cloudy is household dust, which is largely quartz at hardness 7. Anything softer accumulates fine scratches simply from being worn, and rings suffer most.
  • A scratch test always damages one of the two materials. Never run one on a gemstone.