Mohs scale of hardness

Also called: Mohs scale · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

The Mohs scale of hardness ranks minerals from 1 (softest) to 10 (hardest) by how well they resist being scratched. Each mineral on the scale can scratch every mineral below it. It was made in 1812 by the German mineralogist Friedrich Mohs [1].

It matters because hardness is one of the quickest ways to identify a mineral in the field, with no laboratory needed. UPSC often tests the order of the ten minerals, and the fact that the scale is ordinal (it shows only the order, not how much harder one mineral is than another).

Explanation

How it works: the scratch test

• Hardness means how well a mineral resists being scratched.
• The Mohs scale is a rough measure of how well a smooth surface resists scratching or wearing [1].
• Rule: a harder mineral scratches any softer one.
• Scratch an unknown mineral with the standard minerals, one by one.
• Find the softest standard mineral that can scratch it, and the hardest one it can scratch.

• Example: a mineral that scratches fluorite (4) but not apatite (5) has a hardness between 4 and 5.

• Geologists never check hardness alone. They check it together with other physical properties, such as streak, lustre and cleavage.

The ten standard minerals

Hardness	Mineral	Note
1	Talc	Softest; feels soapy
2	Gypsum	Scratched by a fingernail
3	Calcite	Main mineral of limestone and marble
4	Fluorite
5	Apatite
6	Orthoclase feldspar
7	Quartz	Scratches glass
8	Topaz
9	Corundum	Ruby and sapphire are varieties of it
10	Diamond	Hardest natural substance

• Mnemonic: "Tall Girls Can Fight And Other Queer Things Can Do."

Everyday tools for a quick field test:

Tool	Approx. hardness	What it tells you
Fingernail	~2.5	Scratches talc and gypsum
Copper coin	~3.5	Scratches calcite
Glass / steel knife	~5.5	Scratches apatite but not orthoclase
Quartz	7	Scratches glass

The scale is ordinal, not proportional

• Ordinal means it shows only the order (1st, 2nd, 3rd…), not the size of the gap.
• The 10 standard minerals were given arbitrary numbers, chosen for convenience rather than measured [1].

• The scale is not linear. A step of one does not mean the same rise in hardness each time [1].

• Proof from absolute tests:

• Corundum (9) is about twice as hard as topaz (8) [1].
• Diamond (10) is almost four times as hard as corundum (9) [1].

• So diamond is far more than "ten times" as hard as talc.

• Absolute hardness is measured with indentation tests such as Knoop and Vickers. In these tests a tip is pressed into the surface instead of scratching it [1].

Structure decides hardness: graphite vs diamond

• Graphite and diamond are allotropes of carbon (different crystal forms of the same element) [4].
• Diamond: each carbon atom is bonded strongly to four others in a tetrahedral (pyramid-like) 3-D network.

• Result: hardness 10 [4].

• Graphite: carbon atoms form rings of six in flat sheets. The sheets are held together only loosely.

• Result: hardness only 1–2. It is soft and greasy [4].

• Lesson: hardness depends on how the atoms are arranged inside a mineral, not only on what it is made of.

Where it occurs

• Field geology across India: geologists use hardness, together with streak, lustre and specific gravity, to spot ore minerals before costly lab tests.
• Hematite (iron ore): Bailadila (Chhattisgarh) and the Odisha–Jharkhand belt. It is checked by its red-brown streak, and hardness is tested alongside.

• Mica (a soft mineral that peels into sheets): Koderma–Gaya–Hazaribagh belt (Jharkhand), Ajmer (Rajasthan) and the Nellore mica belt (Andhra Pradesh).

• Rocks around the world: calcite (3) is the main mineral of limestone and marble. This is why these rocks are fairly soft and easy to scratch.

• Streak testing: the unglazed porcelain streak plate works only for minerals up to about 6.5 on the Mohs scale. Harder minerals scratch the plate instead of leaving a powder [3].
• Industry: very hard minerals, corundum (9) and diamond (10), are used as abrasives (materials that grind and polish) and in cutting tools.
• Gem identification: hardness is one of the standard properties used to identify minerals and gem stones [2].

Don't confuse with

• Absolute hardness (Knoop / Vickers tests): these measure how much harder a mineral is, by pressing a tip into it. Mohs gives only the order, by scratching [1].
• Cleavage and fracture: these describe how a mineral breaks. Mica splits into flat sheets (cleavage). Quartz breaks with curved, glass-like surfaces (fracture). Hardness describes resistance to scratching, not breaking.
• Specific gravity: this is how heavy a mineral is compared with an equal volume of water at 4 °C [2]. A heavy mineral is not always a hard one.
• Graphite vs diamond: both are pure carbon, but graphite is 1–2 and diamond is 10. The difference comes from their atomic structure (loose layers vs a tetrahedral network), not their chemistry [4].

Prelims Hooks

• The Mohs scale was made by Friedrich Mohs in 1812. It ranks minerals from 1 to 10 [1].
• Correct order: Talc → Gypsum → Calcite → Fluorite → Apatite → Orthoclase → Quartz → Topaz → Corundum → Diamond.
• Trap: "Diamond is ten times as hard as talc" is wrong. The scale is ordinal and non-linear. Diamond is almost 4 times as hard as corundum alone [1].
• Corundum (9) is the mineral of ruby and sapphire. Calcite (3) is the main mineral of limestone and marble.
• Field checks: fingernail ~2.5, copper coin ~3.5, glass or steel knife ~5.5. Quartz (7) scratches glass.
• A streak plate is useful only up to about 6.5 on the Mohs scale [3].

Mains Points

• Hardness helps mineral exploration (GS-III):
• Field tests like hardness and streak let geologists spot ore minerals quickly and cheaply, before lab tests.

• This supports surveys of iron-ore belts such as Bailadila (Chhattisgarh) and Odisha–Jharkhand.

• Hardness decides industrial use (GS-III):

• Hard minerals (corundum, diamond) serve as abrasives and cutting tools.

• Soft graphite suits pencils and lubricants. Its ability to conduct electricity also makes it useful for battery electrodes.

• Structure links to landforms (GS-I):

• The graphite–diamond contrast shows that rocks made of the same elements can weather and erode at very different rates.
• Soft, calcite-rich rocks such as limestone wear away faster than hard, quartz-rich rocks. This shapes how landforms develop in the rock cycle.

Related concepts

• Physical properties of minerals

Read more

• Identifying minerals: physical properties and the Mohs scale